Killer Instinct has two resource gauges that are central to the game. The first is your shadow meter, a purple bar on the lower-left and lower-right of the screen. In earlier versions of Killer Instinct, the shadow meter used to be colored blue, so if you see videos in this guide with blue shadow meter, don't panic!
Shadow meter is functionally equivalent to super or EX meter from modern Street Fighter games, except you are only allowed to hold two stocks in KI. Also, with the exceptions of Fulgore and Omen, there is no use of shadow meter that requires the entire bar to be full; all uses of shadow meter use only one stock at a time. Each stock of shadow meter will flash white when it is full, giving you quick visual feedback.
You build shadow meter primarily by attacking, but also by playing defense. All normals and special moves that hit your opponent or are blocked will award you with a move-specific amount of shadow meter. Interestingly enough, a blocked move gains more meter than a move that hits. Similarly, when you are on the receiving end of a hit or blocked attack, you will build shadow meter, though generally less than when attacking. You do not build shadow meter for whiffed moves, including normals and special moves, and you do not build shadow meter for combo breakers.
Shadow meter builds very fast in Killer Instinct compared to meter in most Street Fighter games. Specials and normals build a lot of meter when used in neutral, but because many of these hits also enter a combo containing more hits, the rate of increase is even more pronounced. Additionally, many characters can choose to build a large amount of meter when they end their combos. Because you can only hold two full stocks, you are expected and encouraged to spend your shadow meter regularly and in creative ways.
There are two primary ways to spend your shadow meter.
Shadow moves are powered-up versions of your special moves. To activate a shadow move, perform the normal special move input, but instead of pressing one punch or kick, press two punches or two kicks. Aside from Cinder's shadow Pyrobomb, it does not matter which of the two punches or kicks you press, as any combination of two punches or kicks will produce the same effect. You can even use three punches or three kicks to execute shadow moves if you prefer, which is conveniently mapped to LB or LT if you are using an Xbox One controller. Each shadow move consumes one stock of shadow meter; if you do not have a stock of shadow meter available, attempting to input one will execute a normal special move instead.
Because you spend a stock of shadow meter to perform shadow moves, they appear quite similar to EX moves from Street Fighter. In reality, they are more like a combination of Street Fighter's EX and super moves. Like EX moves, shadow moves often gain strong additional properties, like added invincibility, faster startup speed, fewer recovery frames, more damage, or added range. Like super moves, shadow moves announce their activation by temporarily freezing the screen, giving the opponent time to plan his defense.
Measuring how fast a shadow move starts up is slightly different than normal moves. Because shadow moves cause a screen freeze, that means startup for a shadow move is broken down into startup before the freeze, and startup after the freeze. I prefer to use a plus sign to separate the numbers; "3+1" means that a shadow move has 3 frames of startup before the screen freezes, and then 1 frame of startup after.
Unlike modern Street Fighter games, where most super attacks freeze the screen instantly and then have a lot of startup after the freeze, shadow moves in KI will often be slow before the freeze, but very fast after. If a move has 0 frames of startup after the freeze, that means your character cannot change his pre-freeze state when the move begins to hit you. For example, Thunder’s shadow Call of the Earth command grab is 5+0 startup; 5 frames of startup before the freeze, but it activates instantly after. If you are in range and haven’t jumped before the move is announced, you are guaranteed to be hit.
There are two important points to note about shadow moves that do not apply to EX moves or super moves in games like SFIV or SFV. First, the vast majority of shadow moves hit 5 times on hit or block with only a small handful of exceptions, and each shadow move will hit with a different rhythm and timing that is important to learn.
Secondly, the shadow move screen freeze does not ignore your opponent’s inputs. In a game like SFIV, any button you pressed during a super or ultra freeze would not be registered by the game. In KI, these inputs will be queued up and executed on the first frame after the shadow freeze, including jumps, dashes, normals and specials. This means executing a counter move is very easy (simply input your move at any point during the freeze), but it also means that the game will not save you if you accidentally press a button or input a dash during shadow freeze. If you notice yourself getting hit by a lot of shadow moves in KI, this may be the cause.
If special moves in KI are good, then shadow moves are even better. Unless a move is fully invincible, like Jago’s shadow Tiger Fury uppercut, it is almost always safe on block at the very least, and will likely have some combination of upper body, lower body, throw, or projectile invincibility added to whatever invincibility the base special move has. Most shadow moves execute so fast after the freeze that they hit you grounded in pre-jump if you try to jump away from them, so you're forced to block. For example, Jago's shadow Wind Kick is lower body invincible, throw invincible, projectile invincible, travels about 60% of the screen in 5 frames, and is +1 on block. Shadow moves can also be used in the middle of a combo, which interfaces in a unique way with Killer Instinct's Combo Breaker system.