Vulcan finger-touching was a ritualistic form of affection among Vulcans involving the index and middle fingers. It was used throughout their culture, including in public, at wedding ceremonies, and during the pon farr. Contact ranged from a simple two-fingertip touch to tracing around another's hand.
If a Vulcan mated with a member of another species, the non-Vulcan would adopt the practice. The Romulans also had knowledge of the ritual. (TOS: "Journey to Babel", "The Enterprise Incident"; Star Trek III: The Search for Spock; ENT: "Home")
The instances in which Vulcan finger-touching was used included the following examples:
While trapped in the Delta Quadrant in 2375, a telepathic pitcher plant made the crew of USS Voyager believe they had returned to the Alpha Quadrant. Tuvok imagined that he was reunited with his wife T'Pel and greeted her in this manner. (VOY: "Bliss")
Regarding the origins of the Vulcan gesture involving touching of two fingers with two fingers, Leonard Nimoy explained in his book I Am Spock that the gesture was not meant to be the Vulcan equivalent of a Human kiss, but rather the Vulcan equivalent of holding hands in public: "The question came up as to what public sign of affection, if any, Sarek and his Human wife would display. Handholding was clearly out, but perhaps finger-to-finger contact of a ceremonial, dignified nature might work. Mark [Lenard] and Jane [Wyatt] took my comments to heart, and came up with the wonderful gesture where Amanda rests her first two fingers lightly upon Sarek’s two fingers. It worked beautifully, and added to the texture of [the episode]." Nimoy also described Vulcan finger-touching as "the beginning of the Vulcan mating ritual", "the Vulcan two-fingers-touching 'embrace'", and "the Vulcan version of foreplay". (I Am Spock, hardback ed., pp. 71-72, 237)
According to Harve Bennett, Leonard Nimoy imagined that, as a seven-year-old, Spock had engaged in some Vulcan finger-touching upon first meeting his betrothed, T'Pring. Nimoy related this concept to Bennett while they were writing Star Trek III: The Search for Spock. (Star Trek: The Magazine Volume 3, Issue 8, p. 31)
While filming the Vulcan finger-touching with Saavik actress Robin Curtis for Star Trek III: The Search for Spock, Stephen Manley, portraying the seventeen-year-old Spock, grabbed Curtis' hands with both of his hands, an improvised gesture intended to portray passion. Leonard Nimoy, as the film's director, strongly approved of this moment, even fighting for it to remain in the movie when the executives at Paramount were eager to edit it out because it was eliciting a lot of laughs from viewers who weren't entirely familiar with Star Trek. Manley was extremely grateful for Nimoy insisting on the scene remaining. At photo opportunities during Star Trek conventions from 2005 onward, Manley would typically hold his fingers touching those of female fans who wanted a picture taken with him. ("Spock: The Early Years", Star Trek: Motion Picture Trilogy (DVD)/(Blu-ray) special features)
A scene in the script of unmade movie Star Trek: The First Adventure involved Spock, as a cadet, teaching a stripper how to do Vulcan finger-touching. The script's stage directions identified this gesture as actually being pon farr.
In the first draft script of ENT: "Fusion" (which had the working title "Equilibrium"), Vulcans T'Pol and Szon did some finger-touching during a dream T'Pol experienced. This finger-touching was to have taken place in Enterprise's mess hall, empty apart from the pair, and was described in the teleplay's stage directions thus: "Szon moves to her, reaching out two fingers in a traditional Vulcan gesture. T'Pol presents the back of her hand to him, and Szon presses his fingers against her flesh… T'Pol reacts to the sensual touch." In the dream sequence, the finger-touching preceded a very sensual mind meld between the couple.
The writers of the film Star Trek debated, among themselves, about the possibility of having Spock and Uhura finger-touching in a turbolift scene from that movie. Ultimately, J.J. Abrams pointed out that such behavior would be unfamiliar and consequently puzzling to a new audience, so the pair of characters instead kiss in the same scene. In a Q&A, Roberto Orci and Alex Kurtzman agreed with this conclusion.