Señor Mark (Gringo in Peru) has a great story on his blog about the dance of the cobrador:
During these stops you can watch the dance of the cobrador. It is like a ballet as the van begins to stop with the cobrador swinging the door open, placing a foot on ground and doing a pirouette, as so not to fall from a vehicle in motion. As a final passenger on the stop gets their last foot into the van, the cobrador will tell the driver to advance. Reversing his spin he will get back on the combi as it is speeding off. Then sliding the door shut and out the window again with his heads and arms: repeating the chorus of the route only to turn around and ask for your fare.
The excerpt above explains specifically what the dance of the cobrador is all about, but the whole article is worthwhile to read as an introduction into the world of combis.