When officially is the ball ready for play?

[waltjp]

Had this happen during a game a few weeks ago.

Team B has taken a time out. A is lined up and ready to go as we're clearing B's coaches from the field. Our U is over the ball instructing A's QB to wait for the whistle. I give the U a nod so he can back out and as he does the QB starts calling signals.

A80, lined up as a slot receiver, starts in motion just as I blow the RFP. A80 hears the whistle and looks to me as he almost comes to a complete stop. The ball is snapped and the QB turns to hand off to A80 but the timing is thrown off because of A80's hesitation. The handoff is muffed and B recovers the ball.

A's coach is upset, claiming that the whistle caused his player to stop and the result was a fumble. I understand his complaint and don't want this to escalate so I move a little closer so I can explain without shouting across the field. I simply told the coach that our U was standing over the ball instructing the QB to wait for the whistle and the QB took it upon himself to start calling his signals before the whistle had blown. The coach appreciated the explanation and thanked me. As I was moving back to the other side of the field I could hear him explaining the problem to his QB as only a head coach who's team just lost the ball could do.