The karate chop is one of the most iconic movements in the art of karate and many other Asian martial arts. The image of a karate master chopping a bad guy on the neck to make him collapse is a part of popular culture. However, what that chop actually does when it hits the side of the neck is more complex than the movies imply. A properly placed karate chop can cause, pain, disorientation and loss of consciousness. It could also have more serious consequences.
A karate chop is a hand strike where the karateka hits a target with the side of his hand, between the top of the wrist and the bottom of the pinkie. This is generally called a sword hand or knife hand by practitioners of the art. A karate chop has two main advantages over a punch. The side of the hand is much less fragile than the knuckles when unprotected, and you can reach different angles -- such as the side of the neck -- that you wouldn't be able to with a punch.
The neck is an important part of your body. It supports the head and contains arteries that supply blood to the brain. It's also very sensitive to impact because it is not protected by bones like the chest, head and face. Getting hit on the side of the neck is much more painful than being hit in the arm or chest. This pain can be enough to overwhelm somebody, especially if hit by surprise. Although the pain alone is unlikely to "take him out," it's very likely to cause at least a second or two of hesitation.
The vagus nerve is the longest of 12 cranial nerves. It plays a crucial role in the involuntary nervous system, controlling body processes such as maintaining constant heart rate. A sufficiently hard and precise chop to the area around the vagus nerve can cause disorientation, dizziness and even unconsciousness.
Along the front section of the side of the neck you will find the carotid artery, the vessel most responsible for bringing blood to the brain. A chop there can momentarily interrupt blood flow, much like squeezing the sides of a garden hose for a moment. Since your brain relies on blood flow to operate, this interruption has many of the same effects as a hit to the vagus nerve: disorientation, dizziness and brief loss of consciousness.
Although it looks like fun in the movies, an effective chop to the side of the neck is serious business. It can knock someone out cold and may cause more serious damage if the blood supply to the brain is cut off, or if there is injury to the spinal cord. It can be very useful for self-defense, and experienced practitioners of karate understand how, when and where to perform the move.