How do you unlock the anti form in Kingdom of Hearts 2?

In Kingdom Hearts 2, Anti Form is unlocked by using the Drive Gauge. To trigger it, you need to use the Drive ability and have a high number of Drive Points, but only when you're in a critical situation. Specifically, if you enter a Drive Form and then get hit while in that form, there’s a chance you may shift into Anti Form instead. This form cannot be controlled and is not available in every battle, so it’s a rare occurrence.