Learn to float your bike over obstacles.
Pressure control is the use of body movement vertically over the bike to make it lighter or heavier as required by the trail. Lighten the bike to ride over short rough sections of trail or weight it up for extra grip.
Think of a Kangaroo bouncing qucikly across the a field. The faster they go the more distance they cover with each leap. As they land from one leap they bend their legs to absorb the landing then use that stored energy to launch themselves forward for the next leap. To lighten the bike use a similar technique to take most of your weight off it as you come to a rough or rooty section.
As you approach the rough section use both your arms and legs to push yourself quickly up and away from the bike. As you stop pushing with your arms and legs your bodies momentum will make it want to continue upwards for a second or two taking your weight off the bike. You should feel nice and light as if on a trampoline. The bike is now unweighted until you need to use your arms and legs to stop yourself falling onto the bike. The more effort you put into pushing your body up the more you can unweight the bike.
You can use this unweighting in a number of ways.
Completely relax your arms and legs for maximum unweighting over a short period of time for short but very rough sections or maintain a little support in your arms and legs to slowly return to the bike. This will give less unweighting but over a longer period of time for longer not so rough sections.
Start and finish in a crouched riding position for even more range of vertical movement and a stronger or longer unweight.
As you return to the bike you will need to slow your fall using your arms and legs. As slow your fall you will feel heavier, you are now weighting the bike. Your downwards momentum is being transfered through your bike to your tyres, forcing them into the dirt giving you extra grip. The extra grip lasts until you stop pushing down on the bars and pedals.
This extra grip can be used in a number of ways, you could give the brakes an extra squeeze whilst weighted up or use the extra grip to get you round a tight corner a bit faster. These techniques are covered in more detail in later topics.
You can weight the bike up from the normal attack position without unweighting it first by lowering your weight to a crouched position before pushing back of the pedals and bars although coming from fully unweighted will give much more weight to the bike.
Keep your balance on the bike by staying over the cranks.
Keep your weight centered over the cranks all the way over the obstacle to avoid an unexpected trip over the bars or the front end getting loose when you land.