The rev limiter is a safety devise installed so you don't blow your motor. It is controlled through the CDI. By putting an aftermarket CDI in this will allow the motor to climb an additional 1000rpm resulting in a bit more mph. All aftermarket CDI's generally raise the limiter by 1000rpm.
Thanks Red I was brain locked on the comment I made earlier I couldn't think of CDI lol I need sleep really bad!!! but I got a roostboost (which only change timing by altering the ignition pickup) on my quad I don't think I even have a rev limit anymore I'm not going to push it to see either.