Have you ever watched the people getting off a subway train, crowded rush-time, listening to a Tango?! (La Cumparsita for instance) > It's quite a sensation! You see people walking in blocks of pure hurry. As you keep watching, and walking through it, the crowd assumes a form of movement that, watched by attentive eyes, show some synchronia moving upstairs. One can imagine the block of tense people executing the steps as the dance goes on upwards, till the last stairs are won and the movement starts to be more of individual performers while people walk through the corridors, till finally reach the streets 'aboveground'.