I’m a high school coach and my strength is pitching.I also am a lefty so my son,who is now a freshman(lefty)has had good teaching. But I am looking for outside ideas to taking away his foot drag;or cut it down. He is in low 80’s and I know with his size he’s got more! Looking for creative ideas outside of the obvious,as I have done a few things already! Plus its good for my son to read it from somewhere else.
I think you are discribing a dead back side.
One concept of the finish is that he’s not using the front side to pull
the backside through. Also have him start from the stride position and
have him focus on turning the back knee in and thrusting the the back foot and knee up casusing a good vertical move.