The key to riding an mtb over rough sections is to stay loose and allow the bike to move under you. Pick a smooth line that avoids the biggest obsticles but keep your turns gentle rather than zig zaging all over the trail. Keeping your weight centred over the pedals will give you greater stability. Use your arms and legs as suspension to seperate your upper body from any smooth bumps, dips and rollable obstacles you have to ride over. These techniques can be used on the flat, when climbing and when descending. There is a an adapted technique to be used when braking on steep descents to reduce the risk of going over the bars.
Lift over raised obstacles.
Rock, roots and bumps in the trail can cause a problem if you hit them at speed. If you don't absorb the bump it can act as a jump and send you airbourne or kick your back wheel up. At the very least it can upset your front/rear balance making controlling the bike harder.
As you hit a bump in the trail allow the handlebars to come towards you keeping your bodyweight centered over the cranks. Once the front wheel is on top of the bump push the handlebars forward and down over the bump. As the rear wheel rises over the obstacle, allow it to come up under you before extending your legs again as it drops down the other back side of the obstacle. Again, the timing of these two movements may overlap on shorter trail obstacles meaning you need to roll the bike over the bump in one smooth motion.
For larger trail obstacles and humps extend raise yourself up on the pedals by extending your legs just before reaching the obstacle. This will give you much more range of movement in your limbs to suck your bike up and over the obstacle.
Push into drops.
As you approach a dip in the trail lower your body slightly just before the front wheel gets to the dip. Use the handlebars to push the front wheel down and away from you into the hole whilst keeping your body balanced over the cranks. Extend your legs to push the rear wheel into the dip to keep your body high.
If you allow your bodyweight to drop into the dip then it makes it harder to get back out again loosing you speed and affecting comfort. As the front wheel gets to the end of the dip pull up on the bars to help it get onto the higher surface in a similar manner to that used to get over raised obstacles. As the rear wheel comes out of the hole allow the rear of the bike to rise back up under you.
For bigger dips and holes, lower your body before enetring them to allow you to extend your arms and legs more through the dip.
Bigger and faster.
Move your weight to a little behind the cranks as the front wheel rises on faster and larger obstacles or mounds which you can't fully suck up. This will give a little extra stability but make sure you suck up the rear wheel as much as you can as it goes over the bump or the bike may buck you up and forwards over the bars. Return your weight to it's neutral position as the bike levels as you extend your legs on the backside.
Rougher downhill sections.
As discussed previously, braking reduces the bikes ability to handle rough terrain. Braking hard on steep and rough downhill sections greatly increases the chances of an over the bars trip to the A&E department of your local hospital. As the trail approaches the limit of your riding ability, control your speed in the smoother approach and then let go of the brakes through the rough section using the smoother trail afterwards to again control your speed.
Downhill rocks and roots.
This is a variation on the technique used for riding over smaller trail obsticles. It takes advantage of the different weight distribution found when braking on steeper sections of trail.
If you are worried that the front wheel will hang up on a particular rock or root whilst braking downhill, release your brake just before the front wheel hits it. Pull on the bars as described above and move your weight forward whilst extending your legs slightly to raise you body. This will momentarily unweight the front wheel allowing it to bump over the rock or root.
Once the front wheel is over the obstacle, gently re-apply the brakes which will weight up the fornt wheel again whilst lightening the rear. As the rear wheel goes over the obsticle, suck it up with your legs and return to your balanced riding position.
For repeated rocks or roots more than a bike length apart repeat you can just repeat this technique.
If you want to learn how to tackle a section where the front wheel hits the second obsticle before the rear wheel clears the first then you need to check out the 'Pressure Control' section.