He's talking about initially loading, or clearing stoppages, by punching the gun forward instead of pulling the slide back.
There's nothing magical about it. It's simple mechanical advantage. Most people can generate more power punching the grip through, while holding the slide overhand, than they can generate by trying to pull the slide.
Pulling the slide isolates to the biceps, forearm, and hand muscles.
Punching through involves core muscles, pectoral, and triceps, while still using some muscles of the weak arm and hand. IE it uses more and bigger muscle groups.