didn't say to knock out with the flick jab from the limp arm but the more power you generate the faster the punch in this case which is what ppl argued about
let's say punches are either forward, to the sides and in, upwards or downwards which are the free ranges of motion for the arm , the more muscles and levers you use to bring the punch in one direction the more power you generate , so for forward you can use any muscles and levers to help you reach farther forward and generate more power, there is a limiting case for speed where if you use slower muscles and larger pivoting arms (larger trajectory) you can loose speed, all this movement is coordinated by your brain which manages the kinetic chain in order to not fire muscles which oppose the motion of the body, the torso is just another lever like the waist e.t.c. which is affected by gravity , really hard to explain every detail , I don't think anyone bothers with each individual movement, most coaches recognize a pattern and copy this pattern which is what we humans are really good at
edit: I now remember there is youtube video where a guy compares mma fighters with big bellies , it describes kind of what I say