Junior Dragster...

Why yes I can. They kill the ignition momentarily to bring the rpms back down. They feel that the clutch starts to grab the belt higher in the sheaves after the burn out and by lower the rpms it resets the clutch back to its almost rest position. If the belt is in different positions at the hit. The launch rpm will be different. You will also see some crew chiefs push the car backwards a few feet after the burn out. It effectively does the same thing.
Some of them were pushing the car backwards...makes sense.

Yes, but the really good ones are more modeled off snowmobile race clutches. When I first started it was all about the "Comet" clutch. We have gone way past the start point. In fact, it has gone so far overboard you could blow way over 10K on a setup without even trying. A new motor (single cylinder flathead mind you) off the shelf was more than a crate (GM) motor. I built and maintained all of my pilots motors and was able to take three track championships over her junior career. Only broke one motor (split a crank) and never windowed one. You want to talk about something that shouldn't do what it does. These things cook at around 9 to 9.5K rpm which should be way out side the power band. What is that? Firing about 60 times a second. They launch quicker than a corvette and can be driven by kids I think down to 5 yrs old now. We traveled from Canada to Colorado to race and had a blast. And now there is a new mix. Electric jr dragsters.
Friend that races a bike has some sort of oddball centrifugal clutch in it. I can pull it in with my pinky. Amount of lock up is determined by RPM. The lockup RPM, if I understood him right, is controlled by weights and springs of some sort. Bike is in the 8.1-170+ range. He leaves at full throttle and just literally dumps the clutch. He runs a 140 shot of nitrous. I just found out that if he does not run nitrous that it runs thru the clutch...making it slip like no tomorrow.