The FSK-401 Sustainer by Fernandes is an electromagnetic system powered by a 9V battery. It magnetically vibrates the guitar strings once you have set them in motion, to give you nearly endless sustain and, in a second mode, to give you harmonic feedback at any volume level. Installation of the Sustainer Kit may require routing.In standard mode will continuously vibrate the note In harmonic mode will create 5th harmonic feedback at any volume level