Power ultimate comes from the ground so to get the best outcome is a matter of torquing the entire body to transfer that force, and that's what he went with, he doesn't punch like a Boxer because even though they do hit very hard, that is a punching mentality and a lot of kicking becomes incompatible, which is why you never see Kickboxers or even MMA fighters punch and kick off the same action.

His main reason for discarding Wing Chun was that it is not dynamic enough to work in a fight, he had little success with it against Boxers and the WJM fight was at a time when he had already combined styles to make his own eclectic system. Centerline principle is what he kept from WC and has a scientific basis, the vertical fist results from keeping the elbow tight on the body, which aligns the muscles correctly in order to deliver maximum force. There are other things involved such as dropping centre of gravity during the motion and in particular initiating the shoulders with the live hand (non-punching hand), then executing everything simultaneously which takes a long time to make impulsive, like anything. Read pages 93 and especially 94, they explain it.