My take is the coach saying to stay behind the ball is referring to avoiding trying to help turn the ball by coming around the ball as you release it.
If you try to stay behind the ball as it falls off the thumb your fingers stay in the ball longer and you put more consistent rotation on the ball.
As far as suitcase release while I'm sure there are many players still using it the only time I use it is shooting at 7 or 4,7 spares where I want the ball to work across the lane.