For those wondering here is the OFFICIAL info on the Anti-form from the Strategy Guide for the game:
"Everytime Sora changes into Valor, Wisdom, or Master Form, there is a chance that he will instead transform in the dark Anti Form. In fact, the more Sora changes Form, the greater the odds of taking on Anti Form become! In this mode, all of Sora's commands are disabled except for Attack and Return to normal. Sora moves at double-speed, but this too comes at quite a few costs. Not only does he take 50% more damage from attacks, but he can't pick up HP orbs to regain HP. Sora also doesn't gain EXP from defeated foes. The Form Gauge depletes faster if you pick up Drive orbs.
The odds of changing to Anti Form are dictated by "Anti Points". Your actions rise and lower your unseen "Anti Point" total, as do the situations under which you change Form! Anti Form cannot be voluntarily entered, does not level up, and does not earn you any new Abilities for normal Sora. Also, you cannot enter Anti Form with an unconscious ally on the field or by changing to Final Form.
CHANCES OF CHANGING TO ANTI FORM:
ANTI POINTS-----CHANGE PROBABILITY
0-4-----------------------0%
5-9----------------------10%
10+---------------------25%
ODDS MODIFIERS:
CONDITION---------------------------------------------ODDS CHANGE
MOST SCRIPTED BATTLES----------------------------------------------X2
BATTLES AGAINST ORGANIZATION XIII------------------------------X4
BATTLE AGAINST ARMORED XEMNAS--------------------------------X10
BATTLES WITH NON_PARTY COOPERATIVE CHARACTERS----------X0
ANTI POINTS:
Change to Valor, Wisdom or Master-------- (+1)
Change Form when ally is unconscious----- ( 0 )
Change to Anti Form-------------------------- (-4)
Change to Final Form------------------------- (-10)
Obtain new Form------------------------------ Drop to 0
" <---- Look I closed the quote, whoot remembered
Well I hope this helps and clears up some stuff.