Third to first pickoff

[bossman72]

Had this in my game the other day:

Pitcher tries the old "third to first" pickoff move in the third inning and does it legally by stepping to third, then removing his pivot foot from the rubber, then making a move to first.

Later in the game, he tries it again, but he doesn't remove his pivot foot from the rubber. I call balk. Coach goes nuts. He wants a rule reference on why this is a balk, and i couldn't recite an exact rule (OBR) but i said he must disengage before making his move to first.


So help me out guys. What rule is he breaking when he does not remove his foot from the rubber?