Used is a noun and verb. This is a move performed at an alternative concert varying from hardcore to even pop punk. To make the "swimming in a pit move" you get on the ground in a mosh or regular pit at a concert and pretend to swim (stationary) by waving your arms and kicking your legs. This move is more for laughs than breakdowns.
Girl: "Dude, did you see that guy get on the ground and swim in the mosh pit?"
Guy: "You mean, swimming in a pit. Yeah that is the future hardcore dancing."