Clap on. Clap off. Well, more like, Clap, pause for half a beat but no longer because otherwise it'll stop hearing you, clap again because you waited too long, clap louder and faster, that didn't work, clap two more times, and suddenly: on.
Source: [The Verge](https://www.theverge.com/podcast/967642/the-clapper-version-history)