Cycling Heroes

I went to the Hasselt 6 day and saw the derny bikes there. Didn't really understand how they comunicated between each other and it seemed a bit strange with one of them generally steaming past everyone at the end when he looked out of it earlyier, almost like the dernyer had a more powerful bike. Do the riders generally use the same dernyer or is it whoever gets picked up by each rider?

they tend to have helmet that look like it have two large vent like the Delorean time machine on the back, which is designed for cyclist to shout into in order to communicate.