If a game has slippery slope, it means that falling behind causes you to fall even further behind.
For example, imagine that every time your team scored in basketball that the opponent’s team lost a player. In that game, falling behind is doubly bad because each basket counts for score AND it makes the opposing team less able to score points of its own. The actual game of basketball does not have this screwy feature though, so real basketball does not have slippery slope. Scoring in real basketball puts you closer to winning but does not at all hamper your opponents’ ability to score.
Slippery slope is another name for positive feedback—a loop that amplifies itself as in a nuclear reaction. Because people confuse the terms positive and negative feedback so easily, I prefer the more descriptive term slippery slope.
Slippery slope is usually a bad property in a game. If a game has a powerful slippery slope effect, that means that when one player gets a small early lead, they are more likely to get an even bigger lead, which in turn makes them more likely still to get yet an even bigger lead, and so on. In a game like this, the real victor of the game is decided early on and the rest of the game is futile to play out (or to watch).
Examples of Slippery Slope
StarCraft and Chess both have slippery slope. They manage to be good games anyway, despite this anti-climactic property.
Chess
In Chess, when a player loses a piece, their ability to attack, defend, and control space on the board is slightly reduced. Sure, there are many other factors in Chess—positioning, momentum, pawn structure—that determine if a player is actually “losing,” but losing a piece does have an effect. Clearly, losing a lot of pieces, say 8, puts a player at a significant disadvantage. It’s pretty hard to make a comeback in Chess, and a game is usually “won” many, many moves before the actual checkmate move.
This is why there are a lot of forfeits in Chess. Good players don’t actually play out the pointless part of the endgame when they recognize the opponent will definitely win. Chess players would say that forfeits being a regular part of the game is fine and not awkward, but it’s a disappointing quality compared to games without slippery slope.
Chess 2
Chess 2 doesn't have the problem of anti-climactic slippery slope situations because of the midline invasion rule. You win if your King crosses the midline of the board. (You can also still win by checkmate.) This means the game ends before one player falls too far down a slippery slope. It's better to be put out of your misery than stuck in a hopeless situation for many turns.
StarCraft (and StarCraft 2)
StarCraft also has slippery slope. When you lose a unit, you are penalized doubly. First, you are closer to losing (having no units at all is so crippling as to be virtually the same as the actual loss condition of losing all your buildings). Second, you are less able to attack and defend because the unit you lost was not just part of a score, but also part of the actual gameplay of attacking and defending.
StarCraft has even more severe slippery slope when it comes to the game’s economy. Imagine that your opponent rushes you (sends an early attack to your base) and you fend it off. Let’s say you each lost about the same value of units in the exchange, except that you also lost one worker unit. In a different type of game, this might equate to being one “point” behind. But in StarCraft, that can be a crippling loss because gathering minerals is nearly exponential. Your opponent is ahead of you in the resource curve, increasing their earnings faster than you are. You’ve fallen down a very slippery slope here, where an early disadvantage becomes more magnified as the game goes on.
In basketball, the score is completely separate from the gameplay. Your ability to score points doesn’t depend at all on what the current score is. You could be ahead by 20 points or behind by 20 points and have the same chances of scoring more points. But in StarCraft (and Chess), the score is bound up with the gameplay. Losing units pushes you closer to loss AND makes it harder to fight back.
Fighting Games
Fighting games don’t usually have slippery slope. In Street Fighter, for example, your character still has all of his moves even when he’s about to lose. Getting hit puts you behind in life totals (in “score”) but doesn’t limit your gameplay options in the way that losing a piece in Chess does or losing a unit in StarCraft does. An unusual example of a fighting game that does have slippery slope is Bushido Blade. In that game, getting hit can cause you limp around or lose the use of an arm. This is extremely rare in the fighting game genre though, and for good reason.
While it might be "realistic" for a nearly dead character to limp, move slowly, and have generally less effective moves, it's not fun. (At least in Bushido Blade's case, this part of the game lasts only a couple seconds, then you lose.) Meanwhile in Street Fighter, comebacks are frequent and games are often "anybody's game" until the last moment. Street Fighter does have some very minimal slippery slope aspects (if you're very near death you have to worry about taking damage from blocked moves which aren't a threat if you have full life), but overall it's pretty "slippery slope neutral."
There is one fighting game that stands out as an exception: Marvel vs. Capcom 2. In this game, each player chooses 3 characters. At any given time, one character is active and on-screen, and the other two are off-screen, healing back some lost energy. The off-screen characters can be called in to do an assist move, then the jump off screen again. The main character can attack in parallel with the assist character, allowing for a wide variety of tricks and traps. The player can switch the active character at any time, and he loses the game when he loses all three characters. But here, slippery slope rears its bitter head. When one player is down to their last character and the other player has two or even all three of his characters, the first player is at a huge disadvantage. The first player has can no longer attack in parallel with his assists, which often means he has no hope of winning. Comebacks in MvC2 are quite rare and games often "end" before they are technically over.
This was specifically addressed in Marvel vs. Capcom 3 by adding the X-Factor mechanic. That's a mode you can activate once per game that powers you up a huge amount for several seconds. It powers you up more the fewer characters you have left. Activating X-Factor when you're donw to your last character is so powerful that you can reasonably hope to defeat one or two of the opponent's characters before your X-Factor runs out.
Fighting games with ring out such as Virtua Fighter and Soul Calibur are especially good at avoiding slippery slope properties. In these games, a player instantly loses if his character is ever pushed out of the ring, no matter how much energy he has. Basically, no matter how far behind you are, no matter how close you are to losing, you always have a 100% damage move: ring out. Long ago, I thought this concept was "cheap" and served only to shorten games while adding little benefit, but actually the threat of ring out adds quite a bit to both these games. Since the threat of ring out is so great, another whole element of positioning is added to the game. A player must fight both to do damage to his opponent, and fight for position to avoid ring out.
Limited Slippery Slope
Fighting games do have a very localized, limited kind of slippery slope that’s actually a good quality. If a game truly has no slippery slope whatsoever at any point, then it can feel like a series of disconnected decisions. It’s interesting though, if a decision you make at one point in a game echoes forward through time, and can influence later moves in the game. The problem is if this influence is allowed to snowball into a greater and greater advantage.
In limited slippery slope, there is a cap on how far you can slip and the effect is temporary. In Street Fighter, getting knocked down (hit by a sweep) does have a bit of slippery slope. You lose health (“score”) but you also have temporary limitations on what your character can do. Your character falls down, then gets up into what is usually a disadvantageous situation. The two things that are important about this are: 1) after the knockdown is over, you regain all your moves and 2) you cannot get doubly knocked down. (The card game Yomi features this same kind of knockdown mechanic, which also has limited slippery slope.)
Hitting the opponent with a sweep does echo forward through time, but this advantage is reset soon after and can’t snowball into “getting REALLY knocked down” because there is no such thing as degrees of knockdown. If you are already knocked down, you can't be knocked down "even more."
Another example is backing the opponent into the corner (the edge of the stage). If you do this, you have a natural advantage because the opponent has fewer movement options. But again, there’s a limit here. Once the opponent is in the corner, they can’t be “more in the corner.” There’s a limit to how disadvantaged they can get.
An even more basic example is anytime you block a move that has a fair amount of recovery. In these case, you recover from your blockstun before the opponent recovers from their move, so you have a few frames to act first. This gives you an advantage because if you both try to do a move of the same speed, yours will win (it will start first). Your good decision to block echoed forward into the future, but the effect is very fleeting. Even one second later, this advantage fades.
So fighting games are full of small, temporary slippery slope effects that actually help the game. And yet, on the macro level, they do not have the real kind of slippery slope, the permanent kind that snowballs until the game ends. Compare this to Chess where you don’t just get your captured pieces back a few turns later.
An RTS Without Slippery Slope
Here’s an idea for turning the full-on slippery slope (usually bad) into the limited kind (usually good). Both players start with the same amount of resources to buy units. When your units are destroyed, your resources are refunded. A delay in the timing of this refund combined with the build-time for making new units means that losing units really is a disadvantage, but that the disadvantage fades over time, similar in nature to getting knocked down in a fighting game. The real-time strategy game World in Conflict does exactly this, but I’ve never actually played it.
My point here isn't about whether World in Conflict is a good game, or even whether the exact refund system stated above is good. It just shows that it is possible to remove slippery slope from an RTS if you try hard enough. Someone very dedicated to that problem could probably come up with an even better way to remove it that results in a deeper game, rather than a shallower one.
Perpetual Comeback
The opposite of slippery slope, I call perpetual comeback. That’s just a more descriptive term for negative feedback. (Also, negative feedback sounds like a bad thing, but it’s usually a good quality in games, so it’s helpful to have a term that doesn’t sound negative.) A thermostat uses negative feedback to keep the temperature of a room from spiraling out of control.
Perpetual comeback, then, is a quality in which being behind actually gives you an advantage. I’d like to draw a distinction between two types of this effect, though. In one, when you are behind, a force pushes on you to help improve your position. An example of this is the Fatboy mutator in Unreal Tournament. In that first-person shooter mod, when you kill an enemy, you become fatter and easier to hit. When you die, you become skinnier and harder to hit. Multiple hits magnify the effect, so if you die over and over you get skinner and skinner. Note that even if you die a lot, you are still losing (your score is not helped), but you do have an advantage (harder to hit).
A similar example is any version of Mario Kart. The further behind you are, the more powerful the items you get. In last place, you can get the powerful blue turtle shell which has homing powers to zero in on the first place racer. Meanwhile, the first place racer gets only weak items.
Advance Wars: Dual Strike on the Nintendo DS has a similar feature. Each side has a powerful “tag attack” that’s tied to a meter. When you get attacked, your meter fills up at twice the rate as usual, so the losing player will have faster access to this powerful attack, giving them a chance to make a comeback.
In all three of these examples, the games have a force that help out players who are behind and hinder players who are ahead. This is generally a good type of force to have, because it makes games closer, and small early mistakes are not crippling. That said, maybe the effect is too extreme in Mario Kart, or maybe it creates strange artifacts such as avoiding 1st place on purpose for most of the race. And the power of the tag attacks in Advance Wars might be too extreme, making them dominate the game. Tuning issues aside, the concept is still sound and when it’s done right, it can make matches closer and more exciting.
Perpetual Comeback Extreme
There is a different type of perpetual comeback that is far more extreme and far more rare. That’s when getting closer to losing doesn’t JUST give you helping hand, but instead actually puts you ahead. I think the best example of this strange property is Puzzle Fighter.
Puzzle Fighter is, in my opinion, the best competitive puzzle game ever made and I felt that way long before I was lead designer of Puzzle Fighter HD Remix. The game seems standard enough—it's one of those games where each player has a basin that pieces fall into. There are four different colors of pieces, and you try to build big, single colored rectangles (power gems). You can then shatter those rectangles with special pieces called crash gems. The more you break, the more junk you drop on the opponent's side. When your side fills to the top, you lose.
Several factors come together to create perpetual comeback (the extreme version!) in Puzzle Fighter. First, each "character" (there 11 to choose from, including secret characters) has a different "drop pattern." A drop pattern is the pattern of colored blocks that a character will send to his enemy when that character shatters blocks on his own side. For example, Ken's drop pattern is horizontal row of red, followed by a horizontal row of green, then yellow, then blue. Every time Ken sends 6 or fewer blocks to his opponent, he'll send a horizontal row of red. Every time Ken sends 12 blocks, he'll send a row of red, then a row of yellow. Since the enemy knows this, they can plan for it. They can build their blocks such that Ken's attack will actually help rather than hurt. There's one catch: when you send blocks to the opponent, they appear in the form of "counter gems," which can't be broken immediately by normal means, and can't be incorporated into deadly power gems. After about 5 moves, the counter gems change into regular gems.
The other very critical property is that power gems broken higher up on the screen do more much more damage (send many more counter gems) than gems broken at the bottom of the screen. So consider what attacking is actually like in this game. Attacks are really only temporarily damaging, until the counter gems turn into regular gems. At that point, the opponent will probably be able to incorporate the gems into their own plans, since the opponent knows your drop pattern. Even if the opponent isn't able to benefit from your attack in that way, they can still "dig themself out" of trouble by breaking all the stuff you sent them. By filling up their screen most of the way you've basically given them more potential ammunition to fire at you. What's more, as they are nearest to death, their attacks will be the most damaging due to the height bonus. Gems broken at the very top of the screen do significant damage.
Puzzle Fighter has the extremely unusual property that "almost losing" looks exactly like "almost winning." Let's say you break a whole slew of power gems and send a large attack at your opponent. Your screen is now almost empty. You're winning right? Their screen is nearly to the top—almost full. They're losing, right? Well, they are on the verge of losing, but they have all the ammunition and they have the height bonus, whereas you have almost nothing left to defend with. In effect, your opponent is both "losing" and "winning" at the same time. Very curious, indeed!
It turns out the best way to play Puzzle Fighter is to not attack until you can make it count. All those little jabs you do just help the opponent in the long run. You've got to save up for a huge, 1-2 punch. You need to send a big attack that almost kills them, then immediately send another attack that finishes them off. 1, 2! The point is that Puzzle Fighter is a high energy, edge-of-your seat game. Your opponent very often has enough attack to kill you, so you have to have enough defense to stop them. Whenever the scales start to tip in your opponent's favor, they have also, weirdly, tipped in your favor as well, in some sense. A game of Puzzle Fighter is never over until the last moment. Comebacks are the name of the game, and the excitement goes to the very last second almost every time.
I liked this comeback mechanic so much that I included it in my tabletop game Puzzle Strike. In Puzzle Strike, you have a gem pile that fills up by 1-gem automatically every turn. You can crash those gems to send them to other players. If you end your turn with your gem pile totaling 10 or more gems, you lose. So the more gems in your gem pile, the closer you are to losing, BUT it also means you have more ammo to fire at other players and you get the benefit of the height bonus rule. The more gems you have (the "higher" your gem pile), the more chips you get to draw each turn. That lets you do bigger combos and enables you to crash even more gems to save yourself.
So Puzzle Strike has a similar perpetual comeback mechanism to the one in Puzzle Fighter. In each case, you're the most powerful right before you lose.
Conclusion
Slippery slope is a force that punishes players who fall behind, making them even more likely to fall further behind. Left unchecked, this makes for matches where the real victor is decided long before the game actually ends, leading to either boring endgame play, or lots of forfeits. While fighting games lack this overall slippery slope, they do have several forms of temporary, limited slippery slope that improves gameplay. This limited slippery slope probably exists in other genres as well, but could be a conscious design choice for future games. Finally, perpetual comeback, the opposite of slippery slope, is a force that helps losing players and puts the brakes on winning players, making for close matches. This property can easily go wrong if tuned improperly, but if done well, it leads to closer, more exciting matches. Puzzle Fighter and Puzzle Strike take this concept to an extreme, by making winning look almost the same as losing.