The leverage generated behind a single punch is dissipated by the head as it moves away. It is simple physics which is not up for debate.
It is the difference between striking a nail suspended from a string with a 20 lb sledge hammer and striking a nail into wood with a small hammer. The small hammer will deform the head of the nail with the slightest blow. All the kinetic energy is transmitted.
You can demonstrate it on yourself quite easily. lightly strike your jaw with one hand/knuckle (not even the effort put into a hard applause clap) and repeat with the same force using both hands. Consider how many times you can multiple the energy of that blow. It takes nothing to break both sides of the jaw.