The brillophone is an instrument you make by cupping your hands and clapping them in front of your mouth. You form a small, circular opening with your lips, hold your breath, and open your throat as though you're going to swallow or yawn.
Air from the hole above your thumbs forcefully enters your mouth and creates an echo. You create different notes and tones by changing the shape of your mouth and lips.
I learned this from my dad, Henry Brill, who was a prizewinning amateur ballroom dancer in Berlin in the 1920s, as well as an amateur entertainer. Where he picked it up I'll never know.