What it is
The campana is a hand bell. The bongocero picks it up in the mambo — the song's instrumental climax, when the singer stops and the requinto solos.
The strokes
Mouth: a loud, open stroke. Neck: a short, dull one.
The mouth on 1 and 3, the neck on 2 and 4 — a stroke on every count.
What a dancer hears
The bell comes in at the song's most energetic part. It's easy to keep the count by: loud strokes on 1 and 3 (5 and 7), quiet ones on 2 and 4.