There's no consistency in how the races are run or what the courses are like...many (most?) don't do le mans starts, just intervals. And some courses are very long, some very short, some are very climb-y, some are technical, some are 85% fireroad.
It's really up to the individual venue...they're entirely different events depending on where they're held. I've done some where they're just the DH course with a short climb tacked on to the front end, and others where you raced a 40% uphill fireroad that then merged with the very end of the DH course for the finish. In some cases, pure XC bikes (well-handled) were a major advantage over DH bikes (obviously) or even trailbikes. But lots of other places are different.
At Snowshoe, it was an interval start on basically a DH course. It was about a ~10 minute race, and the DH guys had a HUGE advantage over the XC guys. there were a few ups, but it was mostly down. I didnt race it, but I watched. Anyone out there that did race wanna shed a little light? Dartman?