When used with guitar, this foot-pedal kill-switch will momentarily silence your guitar if you step on it lightly, and stay silent if you step on it fully. Once it's fully silent, stepping lightly momentarily pulses your guitar on. Simple enough, but the uses are limitless.
You can use it as an on/off for your guitar, or strum a chord with it off and pulse the chord to the beat of the music, or tap it while it's on with some feedback and make your sound cut out like it was sliced up by a DJ, all live, all with your foot, all with no batteries.
You can use this to pulse any sound source such as turntables, microphones, samplers, keyboard, and especially noisy circuit bent stuff, and give it rhythm hands-free.
You may even be able to build it with parts you already have.