What it is
Timbales are two metal drums on a stand — the higher macho and the lower hembra — with bells and sometimes a cymbal. The timbalero plays with sticks.
They came from Cuban danzón and charanga. In a salsa band the timbalero leads the moves from one part of a song to the next.
The strokes
Rim: the stick on the drum's metal shell, a dry click. It's what the cáscara is played on.
High and low: strokes on the macho and hembra heads, for accents and fills — short rolls before a new part of the song.
The cáscara follows the clave, so it differs in 3-2 and 2-3. In cha-cha-chá it's simpler: a click on the shell on 2 and an open hembra on 4.
What a dancer hears
The cáscara is dense and steady through the verse. What helps a dancer most are the timbalero's fills: a short roll at the end of an eight warns that a new part of the song is coming.
In cha-cha-chá the open stroke on 4 is the cue for the "cha-cha".