1. a sexual maneuver named for the aforementioned actor, most notable for his starring role in 'The Karate Kid', and, more specifically, for the karate move he performed in the film's grand finale, known as the 'crane kick', in which he lifts one leg off of the ground, bringing his knee towards his chest, with his arms raised in the air like a deranged spider-monkey.
2. the maneuver is performed when a man and a woman are having sex standing, with the woman's back against a wall, and the man lifts her leg, hooking her knee with his arm and bringing it upwards (lifting both of the woman's legs simultaneously is commonly known as 'The Double Macchio with Cheese', but is not to be tried by amateurs, as it is considerably more dangerous than the regular Macchio)
3. the 'Ralph Macchio' can also be known by it's abbreviated form, 'the Macchio'; or is also referred to as 'the Crane Kick of Fury and Desolation'.
4. for added points while performing this move, you can shout "Sweep the leg, Scottie!", or "Cobra Kai! (then) *grunt*!"
source: John and Justin's Dictionary of Love.