Stolen from Wikipedia:
Also, an electronic musical instrument, such as a guitar or bass guitar, may have a kill switch. It is used infrequently but most commonly in heavy metal music. Contrary to popular belief, a kill switch doesn't break the circuit but works by switching between the hot signal from a pick-up and the ground signal. A typical way of achieving this is (on a guitar with a volume control for each pick-up) by turning down the volume on one of the pick-ups then alternating the pick-up selector switch between that pick-up and one with the volume turned all the way up. They are sometimes used when a singer doubles as a guitar player and isn't playing or sometimes used as an effect during a song. Tom Morello of Rage Against the Machine and Audioslave, and Buckethead (such as in "Jordan") are well known for their use of the kill switch.