Typically, the forefinger & either little or middle finger are jerked upwards or placed above the head like antlers or tree-branches. These represent the wife's open legs, the basal joint of the fingers connote her groin, & the hard nails signify her toes.
An amplification involves pushing the nose, representing a strange penis, between the 2 fingers, as recorded by Morris: "The obscene V-sign as spread female legs: Another popular interpretation switches the symbolism from the male to the female. Here, the sign is read as meaning `her legs are wide apart, like this, ready for copulation.' There is some evidence to support this, from the Arab world. There, there is an obscene gesture in which the forked fingers are jerked upwards several times under the gesturer's nose, so that the tip of the nose protrudes through the V-shape, as if it were the penis being pushed through the legs." ("Gestures", Desmond Morris, Peter Collett, Peter Marsh & Marie O'Shaughnessy. London: Triad/Granada, 1979, p.232)
When the fingers are bent at the upper joint, it signifies the toes curling up in orgasm due to the strange penis.
2) Often, a 3rd phallic finger (fore or middle finger) is pushed between the leg fingers of the Shakh-Dar Gesture: "Horns. To make horns at a person, to put the forefinger of one hand between the first & second finger of the other. See Tarlton's Jests, p.15; Cotgrave, in v. Ciron." - 'A Dictionary of Archaic & Provincial Words, Obsolete Phrases, Proverbs, & Ancient Customs: From the 14th Century', J O Halliwell-Phillipps. London: J.R. Smith, 1847 vol. 1, p.460. Morris explains this: "Here, the forefinger symbolizes the penis entering between the spread legs of the forked hand. The insulting message, quite clearly, is 'Someone else's penis is going between the open legs of your wife, hence you are a cuckold.'... The erect fingers are as far apart as possible & hence symbolise a female with her legs far apart, inviting copulation." (Morris et al, ibid. p.125-6)