Son clave, reverse (2-3). The two-bar pattern that every part of a salsa arrangement has to agree with. In 2-3 the "two side" comes first: beats 2 and 3 of bar 1, then the "three side" in bar 2 — beat 1, the and-of-2, and beat 4. Levine calls this reverse clave; the same five strokes starting from the three side is forward or 3-2 clave.
Listen for the gap: the two side is square and on the beat, the three side leans forward with the and-of-2 and lands on 4. Once a tune starts, the clave never flips; montunos, tumbaos and horn lines are all written to agree with it (the montuno's tied-over note always falls on the three side).
Practice: tap quarter notes with your foot and clap the clave until the two bars feel like one shape, then say "2 3 | 1 2& 4" over it. The piano here simply states the clave in both hands so you can see it on the keyboard lane; mute it and clap along with the clave layer alone.