Well, you don't use it. Your chao do. It is up the the CPU to decide whether or not the chao will use it. You cannot force it to use it.
i think he could do light speed attack on sonic 2 battle
You use it to beat the game easier
Use action replay
Sonic Adventure DX is a popular game across many platforms. It is possible to use Action Replay codes in the game to do various different cheats.
Find a fish and use final slash on it.