In NFL if you stop the forward motion, get pushed backwards, then get downed you get to take the ball from the furthest point you got to before you started moving backwards. This stops the defense from walking the ball backwards.

However, I don't think you necessarily need to stop forward motion in order to down somebody. If the player with the ball goes down on his hands or knees and gets touched before getting up then he is considered down OR if some action (pushing or pulling) of a defensive player causes the ball carrier to touch the ground with his hands/knees that also constitutes a down.

I'm not perfectly up to date with football rules so if I got anything wrong please correct me.