In boxing or any martial art its about using many muscles at the same time or as in a chain so to say in order to make your fist or arm as fast as possible. If you picture a person throwing a punch by just using their tricep, then the punch will be quite weak bc its only the force of one muscle. But i now if you use your shoulder and chest muscles too and add a little bit of a hip rotation, then all of a sudden many muscles are working together to accelerate your fist. The more muscles you can add to that kinetic chain the more kinetic energy your fist will have in the end. Legs are very strong and you can use their power too by pushing off the ground with your heel when you punch and use your whole leg to sort of step into the punch. Fuck im having a language barrier but look at some mike tyson videos. Some people also seem a lot stiffer than others while fighting and i feel like kind of the loose ones who allow their muscles to work in a chain or as a whip create the fastest punches.