There are two deadly punches in boxing. One has to do with the way the head of the boxer who is punched is moving. In boxing, there is constant movement by both fighters, back and forth, left and right, up and down. The movement is a big aspect of defense in boxing. The head of the boxer is therefore almost never stationary.
This deadly punch happens when the head of the boxer is actually moving in the direction the punch is coming from. The more extreme the movement of the head toward the punch, the more devastating the outcome because the movement of the head toward the punch magnifies the force of the punch. This type of scenario can be so devastating that you just see the boxer collapse instantly on impact, wilting straight down to the mat and knocked out — perhaps permanently.
The other deadly punch isn’t a punch at all, but the result of a knockout. If a boxer is hit with a knockout punch and falls directly backward, chances are he will land flat on his back, but when he is falling the head tends to lag the body so that the chin may be almost touching the top of the torso as he falls backward. So when the boxer’s back hits the mat, the head has a violent whiplash motion backward, and the back of the head can hit the mat with tremendous force because of the whiplash. There have been many fatalities in boxing from this precise scenario, and aficionados of the sport, cognizant of this, fear for the worse whenever they see this type of knockout.