The principle behind the feedbackers is pretty simple. In the small box is a piezo disc, wired up to a button that connects it and sends signal down to a small amp, which in turn amplifies that and sends it out to a contact transducer that vibrates whatever surface it’s bolted to. This means that if you touch the active piezo box to the surface the transducer is attached to it generates an direct feedback loop through the surface. Add some scrap bolts into that piezo box and enough room to scrape the piezo box around and you get a semi playable instrument that can generate almost guitar feedback sounding scree. Leave the device on a more sonorous surface with a bit of distance and you get drones. I run one of these on the Plague Rider rig and one of the radiator rig at the moment.