When does a pitch start

Question
I'm 3B Coach of a youth team.  I recently called a squeeze and my BR on 3rd left way too early.  The pitcher was pitching from full windup, he didn't step off and pitched to home.  The batter misses the bunt but the BR slid home easily before catcher can make a tag because he had left so early.

QUESTION:  What would happen if the BR from 3rd slid across home plate before the pitch crossed the plate?  If that happened and the ball was then fouled off, would the BR have to return to 3B?  I know if the runner left before the pitcher actually pitched that is a simple steal...but I guess at what point do you consider the pitch?

Answer
If the runner from third came home and the batter fouled the pitch off, then the runner goes back to third.  As far as when does the pitch start? The first movement from the set position in either the  windup or the stretch constitutes the start, or time of the pitch.