What it is
A bell mounted on the timbales stand. In the verse the timbalero plays cáscara on the drum's shell; in the montuno and the mambo they move to the bell.
It's higher and sharper than the bongo bell, and in the montuno the two play together.
The strokes
Mouth: an open stroke, on every count. Neck: quiet strokes in between, where the cáscara's strokes are. That's why the mambo bell keeps the clave's shape, and differs in 3-2 and 2-3.
In cha-cha-chá the bell is simpler: only open strokes, one on every count.
What a dancer hears
The open strokes on every count keep the pulse; the quiet neck strokes tell you which half of the clave you're in.
In cha-cha-chá the bell strikes every count — easy to keep a slow tempo by.