The XBL Burst - Burst the moment you get hit, even if it's just a raw 5A at the start of the round.
In seriousness, while still starting out you should only use your Burst to escape for bad situations or to prevent a loss. Even more importantly, never use a Burst after an enemy's hit. They will be able to RC or just JC and block your burst if you do. Always wait til you see them performing the startup of a move before you hit Burst.