One of the samba instruments is the “conductor” (also known as the mestre, leader or “the one who is doing the hand signals”!).
The conductor leads the groove by using signals to control what is being played, to bring in breaks, speed up or slow down the rhythm, add or temporarily stop instruments — basically direct the music.
Each instrument has its own signal, as have all the breaks, plus all the quirky extra tricks used to really get the groove going.
The RoR network bands all try to use the same signals so that we can all learn the same tunes internationally, and can play together when we meet up as a big band at demonstrations for example.
The signals indicate what the mestre wants to happen next. S/he will make the signal, then count it in. Signals are never acted upon immediatelly they’re counted in, so all members of the band play the same thing all at the same time.
Signals can be used either individually, or joined together in combinations depending on the effect, the mestre want to achieve. Signals are acted upon in the order they are signalled.
Also remember that some tunes run over 8 beats rather than four, so it is very important to count in at the end of the pattern rather than half way through if you want to prevent general havoc!