Once the pitcher separates his hands, he has committed himself to pitch. Stepping off the rubber alleviates his obligation to pitch as he is now an infielder. Which came first, the chicken or the egg? If the pitcher clearly separates, and then decides to step off, it is a balk. If the pitcher starts to step, but before he completes his step he separates his hands, he’s fine. If he does it at the same time, the umpire should provide some leniency.
OBR rule 6.02 (a) comment: Umpires should bear in mind that the purpose of the balk rule is to prevent the pitcher from deliberately deceiving the base runner. If there is doubt in the umpire’s mind, the “intent” of the pitcher should govern.