On technical singletrack, you need to be able to roll over bumps, absorb unexpected hits, shift your weight around to maintain traction and most of all, know how to pick a line to maximize your speed.
You need to assume a dynamic position that gives you complete control over the bike. This is the stance you should take when you're coasting for extended periods, between bursts of pedal strokes, descending, or any other time it's possible. The idea is to "float" over your bike rather than drive yourself down into it.
Keep your pedals parallel to the ground. A low riding pedal is likely to catch on an obstacle protruding from the trail. But more important, this pedal position provides a solid platform for your feet — a big help when you're floating over the bike.
Your weight should be off the saddle. Your butt should graze the saddle — not quite sitting, not quite out of contact. You're using the saddle more as a guide to feel the bike under you than as support.
If you plant your weight on the saddle, every kick of the bike also kicks you. But don't be so upright that you tire your legs and feet. You should feel like your weight is distributed evenly between the handlebar, pedals and saddle.
While on the trail, you need to plot the best course for your bike not always the smoothest or shortest, but the one that keeps you moving the fastest while expending the least amount of energy. This is known as the "line." It is one of the hardest skills in mountain biking to learn.
Learning the 'line'
Look where you want to go. Our instinct is to fixate on what we want to miss. This is bad because our bikes follow our vision. So if you look at that rock, or that hole, you go there. Instead, aim your vision along the clear path.
Concentrate only on the obstacles that require some technique to get past. Most of the obstacles you see can be simply ridden over.
Look down the trail. If you look directly in front of your wheel, by the time you see objects you won't be able to react to them. On relatively smooth terrain aim your vision 25 to 40 feet down the trail. Adjust to conditions: as little as 10 feet on hard trails, as much as 100 feet or more on fast, flat paths.