The blow that knocked out the boxer in this picture was delivered to the left temple and forehead. As can be seen, the force behind the blow was so great that it created a temporary distortion of all facial features.
The blows shown here occurred rapidly and with great force, and they all caused the brain to slam back and forth within the skull. The greater the speed and force of the blow, the greater the damage done to the brain.
Depending on the size and muscle mass of the person throwing the punch, plus the size of that person's fist, an average swinging punch will exert about 650 to 800 pounds of force on the area of the body hit by the punch. A professional boxer can exert about 900 ponds of force on the target, while a kickboxer's kick can fall in the range of 1000 to 1400 pounds of force. Even the average force of 600 pounds to the head can cause a knockout and do damage to the brain.
When we speak of force in a knockout situation, we're talking about force exerted on the head in one of two ways. The first is called transitional force, and that's the kind of force that causes the head to snap straight forward or backward or directly to the side. The second type of force is called rotational force. It causes the head to rotate on the neck in a turning motion.
Blows that result in one or both of these types of force being exerted against the head are what cause the slingshot motion of the brain inside the skull that can lead to a traumatic concussion and temporary shutdown of electrical impulses within the brain. That shutdown results in unconsciousness, or what we call a knockout.
According to most sources, targeting the chin with an uppercut blow, the jaw with a sideswipe punch, or the temple with a direct blow are the easiest ways to cause a knockout.