his body dynamics are wonderful here. even though he's going backwards, once his back foot plants it becomes a firm anchor to immediately release a punch. at the same time, his upper body angles back to provide just enough space for a compact, short left hook. his upper and lower arm are also angled perfectly to precisely hit that jaw.
if his timing was just off by a split second his counter hook might've just partially landed; off by more than a split second and he's completely out of position for that perfect counter hook. while going backwards, if he had thrown the counter left hook without a firmly planted foot he would not have generated enough power. if he had instead thrown a jab while walking back, not enough power. a straight punch while walking backwards likely would've been too long and slip past his opponent's face. if he hadn't angled back his upper torso, the counter hook would've likely missed. if his upper and lower left arm weren't angled just right while swinging he might have hit the ear or temple which may not have been the "button" for a knockout.
with his opponent rushing in carelessly, he performs all the right positional elements simultaneously with split second timing. bang, good night.