I was thinking about the punching motion you described - first of all, if the shoulder is fixed to staying in approximately the same plane as the torso, rather than the shoulder being extended, then any punch with the elbow straight down will cause the elbow to travel in the arc you described. Regardless of the action of the waist, or changing the angle of the forearm. Just an observation.
Question: Does breathing, and abdominal pressure play into the power behind your punch? If so, how?

Also, when I was playing with it, I think that I've come to the conclusion that this type of punch with the vertical fist involves a greater contraction of the Latissimus (lats) muscle, as well as intercoastals, compared to a Western boxing jab or cross. Those seem to emphasize more of the front delt and pec. Do you agree with this?