If you are being hit by your opponent's attacks and you are in midair, you can perform an Aerial Roll. It is possible to control the direction of where you roll. You will not be able to move for a certain amount of time while you're perforiming an Aerial Roll, but you'll also become temporarily invincible during the Aerial Roll. When being hit by an opponent, there is a period of time where the Aerial Roll is not available and inputting the Aerial Roll command during this time will not do anything.
Pressing a button the moment you are slammed to the ground will activate an Emergency Roll. You're also invulnerable during the Emergency Roll, but keep in mind that the invulnerability ends the moment the roll is completed.
Other ways to wake up are the Back Roll, Neutral Roll, Forward Roll, or a Quick Wake-Up.
A Forward Roll is useful for when you are attempting to switch positions with one's opponents, but as the invulnerability lasts only at the beginning, you should take care not to get hit.
A Backward Roll involves you moving backward as you stand up and has a much greater period of invulnerability than the Forward Roll. Not very useful if you are attempting to gain distance from the opponent while you are in the corner. The later half of the Forward and Backward Roll can be canceled into a Standing or Crouching guard.
The Neutral Roll is similar to the Emergency Roll, except that is performed as a Wake-Up. Like the Emergency Roll, the Neutral Roll has complete invulnerability and is the safest out of all the rolls, but you are likely to come under attack the moment the Neutral Roll ends.
Although the Quick Wake-Up leaves you completely defenseless until the animation ends, it is the quickest way to return to the standing position. The later half of the Quick Wake-Up can be canceled into a Special Attack or a Guard Like the Aerial Roll, there is a period of time during which one cannot perform Wake-Up actions.
Stagger occurs when a player is hit by certain attacks. If you do not recover from Stagger as asoon as possible, you will be completely defenseless. Press any attack button other than D during Stagger to recover from it. The action of recovering from Stagger also works as a Standing Guard. However, you are still vunlerable to Low attacks at this time, so it is best to perform a Crouching Guard if you expect a Low attack and a Standing Guard if you expect a High attack.