The performer wets their finger and starts rimming the human instrument's asshole. When the pressure is right, the instrument farts, producing a pure tone. With practice full songs can be replicated.
I woke up thinking I was being cooked in a giant tub of watered down chili and then realized I shit the tub after I passed out. It smelled awful and burned in my orifices like I was taking a bath in the Devil's Bathtub.