From what I've read you get a chance to turn into anti-form whenever you drive into valor, wisdom, limit and master AND you are in-battle this chance depends on a hidden counter that increases every time you succesfully drive into any of this forms.
- +1 point for driving succesfully
- -4 points when driving into anti form
- -10 points whenever you drive into final form
There are situations like certain specific battles where your chance is 0 by default and others where your chances are increased. Also the Two Become One keyblade's skill Light and Darkness tells you it has a hidden power , this power forbids you from driving into any form except for anti-form or final form, so if you purposely want to drive into anti-form just equip it and try driving into anything that's not final form.
I don't remember exactly where I read this but it was one of the multiple wiki-like pages of KH for sure
