Oh, yeah, it doesn't "pull" the cyclist like with a rope, but rather "pull" them drafting-wise. Each cyclist has a derny, and the derny driver and cyclist act as a team. If the derny driver goes too fast, they'll drop the cyclist (which happens quickly and dramatically, given the speed), but if he goes too slow, they'll lose the race. Winner is still the first bicycle wheel across the line.
Derny races on the road were extremely long distance races, like 500km. I think there's only one left, somewhere-Paris. It's interesting stuff, pacing fast enough to get a gap on your opponents, but not too fast so you get tired - kind of like a time trial, but with a road race at the end. And the moto and bike rider have to communicate perfectly. But, it's not too spectator friendly (on the road). Racers buzz by at motorcycle speed, then it's over. It must be quite a spectacle on the track, but difficult to tell what's going on.
I'm not really an expert on this, just read some articles, so I'm waiting for someone to correct me...