It seems that they take their "Slows" on the 1-beat. The bass line is Half-Quarter-Quarter (sort of like most contemporary ballroom rumbas) and thus it's easy to count Slow-Quick-Quick. Which is the "first" for dancing doesn't matter, the matching of the Slows with the music does. I've had some teachers start an American rumba on 1 others on 3; they both hit a Slow next measure on 1. The British start with rocks on 2 and 3 and the Slow on 4 like a slow mambo. Lots of salsa and timba have a bass line of dotted quarter-dottedquarter-quarter which is harder to follow (like Blue Bayou).