A punch to the jaw causes the head to suddenly spin around. This quick motion of causes trauma to the brain that knocks the recipient out leaving him unconscious. Points on the jaw that are especially vulnerable to knockouts are the sides of the chin, and where the jaw is attaches to the skull.
Acceleration and Deceleration
When the jaw is punched, the head quickly accelerates around. After a fraction of a second, it quickly decelerates as muscles, tendons, and bones prevent the head from spinning any further. The brain inside the skull is floating in fluid. It accelerates slower than the rest of the head. This forces it to crash into the inside of the skull when the head stops.
When the brain slams into the inside of the skull, it suffers trauma. It then bounces off the inside of the skull and slams into the opposite side. This causes even more trauma. Depending on the force of the punch, this can happen several times before the brain comes to rest inside the skull.
Nervous System Response
The trauma to the brain stimulates an overwhelming number of neurotransmitters to fire at the same time. This essentially overloads the nervous system sending it into a state of temporary paralysis. The person who is hit in the jaw loses consciousness and his muscles relax. The injured person falls to the ground with no memory of being hit.
A person who is knocked out by a punch to the jaw suffers a severe concussion. It can take anywhere from several seconds to several minutes to regain consciousness. It all depends on the severity of the brain trauma. In mild cases, the person who is knocked out can shake off the punch with little more than a headache. In severe cases, brain trauma from a knockout punch to the jaw can cause cerebral bleeding and death.