Now I'm curious about the power generation that flexes the knee, rather than extending it. That's new information for me.
It’s in the rooting that happens when we rotate the torso.
In our most fundamental method for drilling the punch, we rotate the torso so that at the pinnacle of extension, the flank faces forward. Then rotate to the other side for the other punch, so the other flank faces forward. The punch drives out as we rotate to that position. This gives us full body rotation and engagement for power.
The rotation is driven from the feet, up the legs, hips, and torso. But what the feet are also doing is rooting into the ground. We don’t just stand on the ground; the feet are not simply a platform between yourself and the ground. We apply pressure into the ground and slightly outward as a way of bracing our stance. It is an active way of standing, not just passively resting on the ground. The legs are working in order to make that bracing solid and strong.
Think about trying to climb up the inside of a rock crevasse, and you brace one foot against the wall in front of you, and one against the wall behind you. You press with your legs to drive your feet against both walls in order to work your way up.
It is the same idea when we brace against the floor, only the angles are less extreme, instead of directly forward and back, we apply leg pressure forward/down and back/down.
Try standing with your feet together in a neutral standing position. Take one step forward with your right foot, maybe slightly longer that your standard stepping distance. Now try driving your feet into the ground as I described. If you are doing it correctly, you will feel your hamstrings engage in your left leg. Glutes and calves as well.
We keep this rooting braced while we rotate from one side to the other, and back again. And over and over.
The muscles of the back of the legs engage in this process, keeping the root in place during the rotation (the soles of the feet pivot on the ground), which provides for a stable base and adds to the power of the rotation and the punch.
It is something that is much easier to demonstrate in person, than to describe in writing. This is also one of the issues I keep in mind when I discourage people from trying to learn via video or some other method that does not include directly working with a teacher. You could watch someone doing this pivot and you could mimic it pretty easily. Except that what I can virtually guarantee is that you will miss the proper rooting that needs to be done with the pivot. That really needs direct interaction with a teacher in order to know you are getting it right. Without understanding the rooting, the method is much less effective.