They use their little gestures and code to tell each other when a table is hot, what to do next and so on, but given that they're doing it discreetly and with the aim of it being undetectable, why do something like cross your arms behind your back? It's a pretty obvious thing for anyone who's watching them. But then again, not a film to be taken too seriously, I suppose...
I think that they over-exaggerated the gestures for the movie and the audience, hence the super obvious eye contact and practically slow motion gestures. That's just what I always thought anyways.
These were all gestures from Bringing Down The House, they probably dramatized in the film for emphasis but the book specifically mentions these exact cues.