Agree! You want to create a fast "vibration" inside of your opponent's body. In order to do so, how fast that you punch out should be also how fast you pull your fist back. I had experience on this once in my life. My opponent punched at my chest, he then pull his fist back, that create a vacuum between his fist and my T-shirt. When he pulls his fist back, my T-shirt was flying toward him.
When you hit on your heavy bag, if you punch fast and
- also pull your fist back fast, your bag will not swing.
- leave your punching arm straight and freeze into the thin air, your bag will swing away.
Here come an interest question. Is it wrong that when you train your solo drill (or form) to extend your punch, freeze into the thin air at the end of your punch?