I ride FS, but I don't approach these any differently than I did with a rigid. Assuming this is like what you encouter a PLRA...
Brake BEFORE the "obastacle." STAND (just get your butt off the saddle), but what you're going to do next is key. While "standing," bring your butt back, maybe even behind the saddle, and lower your upper body. This does three things:
- lowers your center of gravity for greater stability;
- takes your weight off of the front wheel so it can more freely move over obstacles;
- places your center gravity over the right part of the bike for descents (center of gravity should move back while descending), which is mostly where I encouter roots/dropoffs.
Since you're standing, just let the bike freely move beneath you. When it drops off a root or whatever, a relaxed you will naturally let the front end drop. Your hands will extend forward during the dropoff but will come right back when the bike levels out. This is really what it is about--letting the bike move freely.