Yes, there is a rule. But as many have pointed out, very difficult to enforce.
I have often thought that if I wanted to throw a race in Pro Stock (either bike or car) I would launch and run it into the rev limiter in 3rd gear. Everyone will hear it and I would say that it wouldn't shift. People are going to believe what they want, but that would be plausible. And if a driver wanted to do that, he could do it without instruction and only he would ever know for sure.
If a Nitro CC wanted to do it, just set it up to shake or smoke the tires. If I as the driver wanted to do it on my own, I would try to overheat the clutch backing up from the burnout. Jeff probably knows everyway possible.
And while it certainly does happen, I personally don't think it happens as often as some people do.
Alan