[Jim S]

Mike fairly well summed it up. The penalty is tacked onto the end of the very last run by A (when there is no change of possession during the down.)
A recovery with the player on the ground does not constitute a new run. The play ends simultaneous with the player possession so you go back to the end of the last run (where the ball was fumbled, backward pass, etc.).
If there is ANY run after the recovery you start a new run and that's your new enforcement spot at the end of the run.
Take a fumble that goes forward (or backwards), is then recovered by an A player one step before he goes OB. New run. Or he takes a step and falls down. New Run.
See why you want those little WHITE bags on the ground.....