No Conductor
Four drum voices and no master clock. Each one listens to the others and nudges its own timing towards what it hears. Nobody keeps the time; the time keeps itself. Knock a voice off the beat and listen to the band find its groove again. Hold a knock button and that voice drops out; the longer you hold, the bigger the knock. Let go and it comes back accented, quietening as it finds the band again.
spread —tempo —, found by the band
What's happening
Each voice keeps its own beat. After every beat it compares when it played with when the others played, then moves its next beat part of the way towards them and adjusts its own tempo a little. That's all. There is no leader and no shared clock.
Each time you press Start, a new band walks in: four players with their own idea of the tempo, out of step. They settle into one groove at a tempo none of them was given, and it's different every time. Swap two players sends two of them home and brings in two new ones with their own idea of the tempo; the band finds a new groove somewhere between. A knock pushes one voice late; the others lean towards it a little as it leans back, and the band re-forms somewhere new. Every voice plays louder the further it is from the others, so you hear the knocked one arrive with an accent and settle back to normal as it rejoins.
The rule is the linear phase and period correction used to model ensemble timing (after Vorberg & Wing, and Wing and colleagues' work on string quartets). Two grooves: Rock (kick on 1 and 3, snare on 2 and 4, straight hats, a tresillo on the clave) and Cumbia (kick on the beat, the güiro's long–short–short scrape, snare on 2 and 4, cowbell on the off-beats). Switching groove changes what the band plays, never who keeps time.