Instruments» "Slap" technique on bass guitar

On bass guitar, "slap bass" usually refers to a percussive playing technique most commonly used in funk, disco, soul, jazz, Latin, pop, and many other genres. The style sounds much more percussive than regular fingering of notes with the plucking hand, and is also usually louder (although on an electric instrument, the volume can be adjusted with the volume knob or through compression) and more distinct than the sound of a bass guitar played with the usual plucking techniques. The slap sound comes from the combination of two elements: striking the string with the side of the bony joint in the middle of the thumb, a harder surface than the pads of the fingers (used in plucked fingering); and intentionally allowing the vibrating string to come into contact with the metal frets, producing a "toney" or buzzing sound that is normally avoided in plucked/fingered bass.

In the slap technique, the bassist replaces the usual plucking motion of the index and middle fingers with "slaps" and "pops". In the slap, the bassist uses the bone of the thumb joint to strike the lower strings (usually the E and A, and occasionally D, strings) near the base of the guitar's neck. In the pop, the bassist will use the index and middle fingers of the plucking hand to snap the higher-pitched strings away from the body of the bass, causing them to bounce off the fretboard; this produces a prominent buzzing tone with a sharp attack and more high-frequency vibrations than present in plucked bass. The bassist can play many notes quickly by rotating the forearm, alternately slapping and popping: during the pop, the hand moves away from the fretboard, "winding up" or getting in position for the next slap. The slap and pop techniques are commonly used with pull-offs and hammer-ons with the fretting (usually left) hand, to further increase the rate at which notes may be played. Ghost notes, or notes played with the string damped, are also commonly played in slap bass to increase the percussive feel of the technique.

Source: ~wikipedia