Physiology
The pinch to the subclavian artery has been compared to the karate chop which was used in other 1960s television series to render opponents unconscious.
Over the years, fans and Star Trek Expanded Universe writers have made a number of suggestions as to how it would work.
The book The Making of Star Trek by Stephen E. Whitfield and Gene Roddenberry offers a simple explanation: the pinch blocks blood and nerve responses from reaching the brain, leading to unconsciousness. How this might lead to instantaneous unconsciousness is not explained. (Preventing all blood flow to the brain can result in unconsciousness, but many seconds later.) In this earliest of Star Trek reference books, the pinch is referred to as the “Spock Pinch.”
Another conjecture is that it can be done by applying strong and surgically precise pressure over baroreceptors of the carotid sinus at the base of the humanoid neck. The objective would be to elicit the baroreceptor reflex as the receptors detect an apparent high pressure state due to the externally applied force and causes reflex bradycardia and/or hypotension, leading to decreased blood supply to the brain and syncope.
A third conjecture is paranormal rather than medical: because of Vulcans’ telepathic nature and incredible control over their own bodies, they are able to send a burst of neural energy into another being and overload its nervous system, rendering it unconscious (although the pinch does not work on all species, nor on the time-travelling human Gary Seven). This was supported by the fact that Dr. McCoy could not use it in Star Trek III, but it has been rendered moot by the fact that many non-telepathic characters used it in later Trek series, such as the android Data.
The canonical mechanism of the nerve pinch was finally offered in the episode "Cathexis" of Star Trek: Voyager. There, the Doctor inspects a crewmember who was found unconscious and observes an extreme trauma to the trapezius neck bundle, "as though her nerve fibers have been ruptured"; and it is later revealed that the person was the victim of a nerve pinch.
Read more about this topic: Vulcan Nerve Pinch
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