Originally designed to filter out noise in radio transmissions, phase shifting came into popularity as an effect for musical instruments in the early 1970s with the introduction of the Maestro PS-1. Since then, Phase Shifters have been used extensively to imitate the swirling, airy effect of a Leslie rotating speaker cabinet and their shimmering, liquid quality helped define the sound of Funk guitar.
PS-3 PHASE SHIFTER page at Guyatone 's website...