What it is
Trombones are the low voice of the horn section. They gave 1960s–70s New York salsa its heavy, dense sound: Eddie Palmieri's and Willie Colón's bands were built on them.
The strokes
The trombones play chord notes, short hits as a section, and the push — a chord on the & of 4 that pulls into the next bar.
What a dancer hears
In the moña the trombones answer the trumpets in the second bar. In the mambo they play a riff through both bars, pushing on the & of 4, while the trumpets punch out the clave over it.