[Mike Simonds]

I've only called it once in 9 years of officiating.

Its an easy call, usually for the referee because he will be trailing the play, if the offensive player actually puts his hands on the back of the runner and pushes him forward.

As you note, the usual case is that there is a mass of players surrounding the ball carrier and normally you will have an offensive player continuing his block on the defender and the runner just happens to be in the way.

Actually, I've had several fouls by the offense for blockers coming into the group of players late and blocking a defender in the back or a personal foul. And of course there is the runner being held up by one or more tacklers and the next tackler drives his helmet into him, etc.

So assisting the runner is one of many fouls we need to be alert for.