Does anyone know how gamelin music (Indonesia, Bali, etc.) is created?

I've been to a couple of gamelin concerts - they seem to have between 30 and 40 players, playing the same phrase for a long time (it's got a hynotic quality to it) and then the harmonics will change and the players all move to playing something subtly different. How is this actually done? Is there a 'leader' who signals 'time for a change' and exactly what sort of change it'll be? Watch a gamelin performance and it's difficult to percieve that they're all individual players, so any information on how it's done will satify a long felt want.