Human beings have limits on what they can react to, and this limitation is constantly being strained in fighting games. Killer Instinct brings the issue to the forefront quite often, due to the strong offensive tools employed by most characters and the nature of combo breakers. It is very important for you to understand these limits, as it may change how you approach the game.
The average human reaction time, if asked to simply press a button on reaction to exactly one easily-identifiable stimulus, is around 265 milliseconds; feel free to try it for yourself. Killer Instinct runs at 60 frames per second, so 265 milliseconds converts to approximately 16 frames. This means that if you were waiting for exactly one particular move from your opponent, and only had to adjust your input barely on reaction to that move (for example, changing your block from crouching to standing, or pressing one button), it is possible to react to moves that have 16 frames of startup or more quite consistently. It also means that fighting game designers can design moves around this reaction limit, in order to make certain offensive tools more or less feasible to react to. You can look at the startup for any move in Killer Instinct by going into training mode and turning on Attack Data.
Of course, this magical 16 frame target is a theoretical number, and it's not that simple in practice. Let's take a look at why reaction in fighting games is harder than that.
- Not every move gives a strong visual indication immediately. Even if the frame data for a move indicates it has 30 frames of startup, if the first 20 frames has very little movement, then it will be impossible to react. This means moves that have an obvious visual or aural windup tell are easier to react to.
- Your needed reaction to every move is not the same. The fastest possible reaction in a fighting game is changing from a crouching block to a standing block (moving your joystick from to ) or jumping, because your hand is always on the analog stick and the input change is subtle and straightforward. Pressing a particular button on reaction to a move also sounds easy, but in practice it tends to be a little slower, because your finger might have to move a short distance to the button and your mind might have to briefly separate which button you need to press. The slowest reaction is inputting a special move that requires joystick movement and a button press, like a dragon punch. Because the movement itself takes several frames to execute, you will need to react considerably faster to accommodate this.
- You are almost never reacting only to one stimulus in a fighting game, unless you have a prediction, termed a "read", that your opponent is going to do a certain action. In the neutral game, good players will often crouch block by default, and then try to react to overhead attacks by changing to a standing block. Many characters have overheads that are slower than the reaction limit, so it seems like it should be easy to react! But these characters also have more tools from that range. They may, for example, throw you, continue to do low or mid-hitting attacks for a while longer, or move around the screen (walk backwards, forwards, or jump) which changes what moves you need to react to. If you are focused on reacting to the overhead, there are two possible outcomes: you may get hit by other attacks in the meantime, or you may accidentally flinch to a move that "looks like" the overhead, but wasn't.
So, what does this mean in the context of KI? Let's take a look at a few situations in the neutral game involving moves that are near (or slower than) the reaction limit. Moves that are faster than the reaction limit (such as most normals and throws) are not discussed here, because they are simply unreactable. In order to defend against them, you must act pre-emptively.
Sabrewulf's Jumping Slash overhead is incredibly hard to react to. You execute the move out of a running attack, which also has a low option (Hamstring), so let's say you crouch block by default and try to react to the overhead. Jumping Slash has 15 frames of startup, which is already on the low side of the reaction limit, but it also doesn't have a particularly obvious visual tell until the move is almost ready to hit you. This means Jumping Slash is designed to be mostly unreactable to the average gamer; you must predict it is coming in advance and block pre-emptively (fortunately, it is also punishable if blocked). Jago's overhead command normal has 19 frames of startup, but it is also important to note that Jago raises his sword above his head within the first few frames; this strong visual tell indicates that the overhead, and only the overhead, is coming next. This move is reactable, but it is still quite close to the reaction limit, so it is more realistic that you will not correctly block it every time. Glacius's Shatter is an unblockable attack, and one possible way to avoid it is to jump. It starts up in a reasonably slow 27 frames, which is well past the reaction limit. If Glacius is far away from you, such that Shatter is basically his only threat, you will find it quite easy to react and jump every time. However, if your mind is overwhelmed by close-range Glacius mixups, you may notice yourself unable to react in time, where Glacius has several threats at his disposal and your brain is unable to separate the stimuli. Lastly, we have jumping attacks. Most jumps in the game last about 45 frames from start to finish, giving even absolute beginners more than enough time to block jump-in attacks on reaction. As you become more skilled at fighting games, you will notice yourself able to react to jumps with anti-air special moves as well!
What about combo breakers?
Heavy auto-doubles tend to be the slowest attacks in the combo engine, giving you around 40 frames to recognize and break. This means breaking heavy auto-doubles is about as hard as anti-airing with a normal, and most heavy auto-doubles also have a fairly recognizable visual and audio tell that becomes familiar as you play more matches. While even very good players will sometimes react incorrectly to heavy auto-doubles (so don't feel bad!), it is expected that you can train yourself to react to them most of the time (although sometimes, you may not want to).
Medium auto-doubles take between 30-35 frames, but still in general have a recognizable visual tell. However, you must learn to separate the visual cue of the heavy auto-double from the medium auto-double, which takes a bit of practice and can still trip up good players in the middle of a heated match. A skilled player, however, is still expected to be able to react to medium auto-doubles more often than not.
Light auto-doubles are a different story. Between their first visual cue and the end of the combo break window, you only have about 20 frames to react. This is only possible to break on reaction if you have a strong read; most of the time, you won't be able to react to light auto-doubles. And manuals are completely out of the question; they are often normals with between 5-9 frames of startup and a couple extra frames thrown in to help increase the timing window. In general, manuals are well below the reaction limit and in order to break them, you are expected to notice patterns and break by prediction.
Linkers are interesting, because each character's linkers behave slightly differently. Most heavy linkers are reactable, though, because there will be some unique aspect of the animation or the audio that gives away its strength early, giving you ample time to break. However, many medium linkers are designed to look like light linkers until it is too late to reliably react to the difference. This means, with only a few exceptions, breaking light and medium linkers requires a fair bit of prediction.
And to make matters worse for would-be combo breakers, most characters have more offensive combo tools than just auto-doubles, manuals, and linkers. It is common in Killer Instinct for you to reset your opponent by intentionally dropping the combo and performing one of the aforementioned neutral game mixups. So, not only do you need to attempt to identify the various pieces of your opponent's combo in a relatively small reaction window, but you must be on guard for resets and think about changing your block or jumping in the neutral game. All these aspects added together is why combo breaking is hard, and why you should temper your expectations when it comes to your success rate of breaking combos.