This term "staying behind the ball" has multipul meanings.  During the armswing (which completes at the ankle in the downswing) you should always "stay behind the ball". From the ankle forward (in the follow through) "staying behind the ball" produces more up and down roll favoring less hook, earlier roll and more arcing. Once the thumb clears (you should be behind the ball at this point) rotating the hand around the side of the ball faster and later during the follow through produces more hook, longer length and more skid. The amount one "stays behind the ball" during the follow though is subjective to the bowler, condition, equipement and reaction you are attempting to achieve.  I hope this helps