Turn Audio on and allow the mic. This phone plays a steady click; the mic listens for other pulses nearby and gently adjusts speed and timing to line up — like fireflies syncing.
Set a starting BPM, tap Start, and play in a circle with other phones. Louder speakers help the mic lock on.
Turn on Visual flash for a full-screen pulse on each beat (stronger on downbeats) — useful when you cannot hear the click.