(covers information from several alternate timelines)
The Vulcan nerve pinch was a martial technique developed by the Vulcans. It involved applying pressure near the base of the neck, at the shoulder, and nearly instantly rendered the target unconscious, often so fast that the target was unable to cry out, but not always. (TOS: "Day of the Dove") Being able to perform it was seen as a mark of true Vulcanhood by some. (TAS: "Yesteryear")
The technique did not appear to cause permanent injury and seemed to be effective on most humanoid species. The only Human to have ever been insensitive to it was Gary Seven. (TOS: "Assignment: Earth") When used on Human Augments like Khan Noonien Singh, it caused pain but was not enough to subdue them. (Star Trek Into Darkness) It was proven ineffective on robotic androids when Spock took the time to softly try it on android Alice, who simply asked him calmly if that gesture had any significance. (TOS: "I, Mudd") Vians were unaffected by the nerve pinch. (TOS: "The Empath")
Spock once performed the Vulcan nerve pinch on a horse, Selek did one to a Le-matya (prompting young Spock to ask if he thought he'd ever do it as well as Selek did, which the older Vulcan affirmed), and Tuvok once did it to a member of Species 8472 that had disguised itself as a Human. (Star Trek V: The Final Frontier; TAS: "Yesteryear"; VOY: "In the Flesh") Spock was also known to use a two-handed variety of the technique to subdue two opponents, an [Andorian Elba II inmate|Andorian]] and a Tellarite. (TOS: "Whom Gods Destroy")
Use by non-Vulcans
On some occasions, non-Vulcans have been instructed in the technique. Spock attempted in vain to teach Kirk. On planet Omega IV, while fighting Cloud William and Sirah in his cell, Kirk expressed his appreciation for the neutralization of Sirah by Spock with the pinch. (TOS: "The Omega Glory")
Overall, the nerve pinch seemed to be extremely difficult to learn for non-Vulcans, although the android Data was able to master it, as was Jean-Luc Picard, years after mind-melding with Sarek. (TNG: "Unification II", " Starship Mine", Star Trek Nemesis)
Jonathan Archer was also able to perform the nerve pinch while carrying the katra of Surak. (ENT: "Kir'Shara") Doctor Leonard McCoy, however, was unsuccessful in performing the technique on a security officer while carrying Spock's katra. (Star Trek III: The Search for Spock)
Background information Edit
The nerve pinch was devised for the episode "The Enemy Within", in which it is performed on a duplicate of Kirk. (The Star Trek Compendium, 4th ed., p. 34) In the final draft and revised final draft of the episode's teleplay, the moment of the nerve pinch's first on-screen appearance was described by saying that Spock "kayoed" Kirk's double, and a later line of ultimately unused dialogue had McCoy refer to the double as having received a "rap on the head."
The Vulcan nerve pinch was thought up by Leonard Nimoy, who felt that Spock was too dignified to render someone unconscious by striking them over the head. (Star Trek Encyclopedia (3rd ed., p. 550)) When Nimoy was pitching the idea for the neck pinch, the episode's director, Leo Penn, asked about it and Nimoy said that Spock was a graduate of the "Vulcan Institute of Technology." (He had probably conflated this with the Vulcan Science Academy, which Spock was not known to have attended.) There, as Nimoy explained to Penn, Spock had taken a number of courses on Human anatomy. Nimoy also told Penn that Vulcans emitted a kind of energy (he never specified what kind of energy it was, but presumably he meant bio-electrostatic energy, which was probably also used in mind melds) from their fingertips, which, when applied to certain points on a Human's neck, renders the Human unconscious. Though Penn apparently had no idea what Nimoy was talking about, when Nimoy explained it to William Shatner, the latter understood the concept immediately. Thereafter, Nimoy credited Shatner's reaction for having sold the idea of the neck pinch. (25 Year Mission Tour video)
The Vulcan nerve pinch became known as the "Famous Spock Neck Pinch" (or "FSNP") to the TOS production staff. (Star Trek Encyclopedia (3rd ed., p. 550)) This term (and abbreviation) was used to refer to the technique in the scripts for the series. (Star Trek Concordance, Citadel ed., p. 166) In the final draft script of "The Naked Time", the nerve pinch was referred to as "the famed Spock hand-pinch." In the final draft script of "Dagger of the Mind", it was identified as Spock's "famous squeeze at juncture of neck and shoulder." In the final draft script of "The Menagerie, Part I", it was referred to as "the famous 'Spock pinch'."
Robert Hewitt Wolfe commented that Odo's use of the Vulcan nerve pinch in "Paradise Lost" was scripted by the writers to avoid having to spend money on Changeling morphing special effects in a scene where Benjamin Sisko is freed from detention. Rene Auberjonois commented that he did not know where Odo had learned the Vulcan nerve pinch, but that "Odo is a man of many talents." (Star Trek: Deep Space Nine Companion, p. 303)
In the novelization of Star Trek IV: The Voyage Home, author Vonda N. McIntyre had Admiral James T. Kirk performing a Vulcan nerve pinch to stop one of the 20th century Earth doctors, who tries to prevent Dr. Leonard McCoy from doing a high-tech, non-invasive repair of Pavel Chekov's middle meningeal artery in a hospital scene in that movie. In the novelization, after Kirk successfully uses the nerve pinch, he says, "That never worked before, and will probably never work again."
In the Star Trek RPG, published by Last Unicorn Games, the nerve pinch is part of the martial art Taroon-Ifla, the only known martial art which consists of a single advanced technique. Presumably, Taroon-Ifla includes nearly endless variations and applications of the nerve pinch (the Taroon), making it effective on an assortment of lifeforms and includes less aggressive functions, such as healing techniques similar to acupressure.