The rougher the trail, the more important it is to relax and let your bike do its own thing. If you don't—if you hold on with a death grip and stiff arms and legs—you're more likely to get in trouble.
The typical profile of a mountain singletrack is often mined with rocks, roots and washboard ruts. The surface is a ragged edge of sharp angles and miniature cliffs. When your tires hit these, they bounce. The magnitude of the bouncing is partially controlled by the tires: The greater their width and the lower the pressure per square inch, the more shock they'll absorb. For a graphic demonstration of this, try riding a rocky trail on narrow tires inflated to maximum pressure.
Nevertheless, tires can eliminate only part of the shock. Despite the cushioning effect of tires and suspension, every bump in the ground line is still reflected by bumps in the handlebar and bottom bracket lines. The magnitude is less, but the shock is still present.
The remaining energy, minus a small amount absorbed by the frame and your front suspension, is passed to your hands, feet, and seat.
Consequently, if your arms and legs are stiff with tension, your body becomes the shock's final resting place. You feel like you're sitting on a runaway bronco. Soon, your bike is bouncing off the trail and there's hardly anything you can do about it. Your body is frozen, and you're doomed to crash.
For a smoother, more controllable ride, hold the bike with relaxed muscles. Think of your arms and legs as a sophisticated and highly efficient independent suspension system that enables your body to float over the ground.
If you're riding relaxed, it will have no abrupt changes or movements. Your body's highs and lows will be linked by smooth, shallow curves instead of sharp changes in ups and downs of the bicycle.
Obviously, the only way your torso can remain calm in the midst of such surface irregularities is if the connections between it and the bike are flexible and absorbent. In other words, your arms and legs must contract and expand as the bike bounces. When you're on a trail covered with rocks, for example, let the front wheel bounce up by relaxing the arms, then press it down.
For this to work, you must be off the saddle. Otherwise, every time the rear tire hits a bump—even with a rear suspension—the saddle will bounce you upward and threaten your balance. Don't even worry about the rear wheel; get the front wheel through and the rear will follow. If necessary, you can moderate its bouncing by squeezing the saddle with your thighs. Light pressure will keep the rear wheel's ricochets from getting out of hand.
The goal is to make a stable platform of your torso, the place where your mass is concentrated. Stabilize it and you won't have to worry about the bike, because it's under control when you're under control. Use your arms and legs to make it smooth by lifting and pressing in anticipation of ground contours.