Tapping is the technique used by Eddie Van Halen, Steve Vai, Kirk Hamett, and so on.. It involves hammering on the fretboard with a finger from you picking hand (usually the 1st or 2nd finger) to produce a note. Usually this is followed by then pulling off to notes fingered by your fret hand. The involvement of the picking hand in this fashion is called a tap. It is usually represented by a “T” in tablature.