Ok here's is something that has been driving me crazy for years. The point of tha Yama Zuki. I looks like the hand in back is used as a overhand which is a punch that I make a LOT of use as someone who practices and spars in the form of kickboxing taught in my dojo. It seems like an easy punch to use with a fair amount of power but the lower hand still perplexes me.
One possible theory I constructed when a friend of mine was showing me Kempo. He had me stand still and take a couple hits only to find the way my body buckled from each specific impact in disturbingly consistant/entertaining ways. The light bulb went on above my head when he did a downward strike to my stomach, causing my head to come downward but mostly forward.
Considering how my head gave forward momentum to add to the momentum of my attackers fist to my face, could the lower hand of yama zuki be used to cause the forward buckling reaction? And if we consider reaction force, my being hunched over thus my hed being in front of my first vertibrae, could this put me in the position to not be able to roll with the punch?