From the Kingdom Hearts wiki
Anti Form appears when one tries to activate a Form, the possibility of it appearing depends on the Anti Points (a hidden counter, which increases with each Drive by one point except by Anti and Final Forms). It is the only form where Sora is without his Keyblade. It specializes in blindingly fast and feral attacks and intense combos, but lacks the ability to cast spells, protect against attacks, recover HP, or use Reaction Commands.
