it's like riding over a log on a mountain bike. the people who were clearing them cleanest last year were doing something like this:

http://www.youtube.com/watch?v=NnMmi...eature=related

your rear wheel needn't clear the log entirely but it's tall enough that you'll want some upward momentum on the rear.

personally, if i didn't have any experience doing it i probably wouldn't (and didn't) try it in a race.