Trilero is one of the Spanish names for Thimblerig performers. They used to be very common in Las Ramblas, where they would set up their little tables and try to hook pedestrians (or more commonly, tourists). I don't see many nowadays, and in a way, I miss them.
They always work at least in pairs, with one watching for the cops. I believe that sometimes they work with pickpockets, so watch out, you might lose more than your bets.