Help with "option" plays controls

I have been practicing option plays and they make very little sense, for the following reason:

With some of the option plays, you can hold the A button down and the QB will keep the ball.  But with other option plays, holding A does nothing and the QB hands it off no matter what unless you dump it fast.

How are you supposed to know when the QB can keep the ball and run or pass it, and when holding A won't work?

As far as I can see from the play descriptions and illustrations, there is no way to tell.

Thanks for the help.