As RacerEx said, MTB races are really sloppy TTs, with some yelling thrown in.
However, as I've discovered through trial and error, the difference between a road TT and a MTB race is that a hard start in an MTB race won't necessarily kill you, whereas it will in a road TT.
The best races I've had have been the ones where I go out with the pros, go until I blow, and then hang on for dear life. Having other fast people to ride with will not only help you pace yourself at a level you probably wouldn't be able to achieve solo, and it will show you the fast lines necessary to pick your way around the course in the quickest possible manner.
The worst races I've had were the ones where I decided to pace myself at the start, and had to pick my way through much slower competition. Not only were they getting in the way, but they were taking sloppy lines and kept me from getting up to a nice cruising speed.