C-C-C-C-C-COMBO BREAKER!!
Breakers allow you to escape your opponent's combo at the cost of 2 bars of your X-Ray Meter. Breakers will cause you to come out of Hit Stun and strike the opponent and knock them down. However, the attack from the Breaker causes no damage to your opponent at all. You do gain positional advantage, as they are now grounded, but that's the only benefit you get besides escaping any more hits in the combo.
To perform a Breaker, hold Forward on the controller and, right as a hit is about to make contact with you, press the Block button. You'll actually counter the move before it hits you so you don't take the damage from the move that you performed the Breaker on. Because they actually interrupt the move before it hits you, Breakers actually do require a small amount of timing. If you are just tapping Block to trigger a Breaker, the timing is similar to Parries from Street Fighter III: Third Strike, though the window is a little larger. If you do not press the Block button right before a move makes contact with you, you cannot break the combo until the next hit.
However, you can actually perform breakers without any timing at all. You can literally just hold Forward on the controller and hold the Block button down. The game is simply checking to see if Block is down and Forward is being held when you get hit. If so, the Breaker triggers. So once you start getting hit by a combo that you want to break, you're actually better off holding Forward and Block and just letting it happen. Of course, the downside to this is that you'll end up performing Breakers by accident from time to time, which can be frustrating.
The Limitations of Breakers
One important thing to note about Breakers is that they can only break combos. If the opponent hits you with one attack, you cannot perform a Breaker. Breakers only work on the 2nd hit or later of a combo. Also, X-Ray Attacks are an exception to the rule: the first hit of an X-Ray Attacks cannot be broken, so if the opponent activates their X-Ray Attacks, you can no longer escape the combo.