When does the trailing arm become unglued and the punch begin?

Tom, Great content, well organized and well presented! My question is at what position does the trailing arm separate (unglue itself) from the body. Perhaps it is at the lowpoint in the downswing or could it be later at impact? I also wonder about the trailing arm punching motion. It seems the trailing arm must unglue to initiate the punching motion that you mentioned. Also, I notice the arms are both straight after impact. It would seem the straightening of the trailing arm is a result of the punching motion. Any help on these questions would be greatly appreciated. AJ