The key to technical down-hilling is to relax your upper body. The steeper and rockier the down-hill, the more tightly the rider grips the bar. A rider will slow down as they approach an obstacle, say a rock, applying both brakes as they go.
 

However the rock will try to stop your wheel. If you have your brakes on, the brakes will try  to stop your wheel, (I know this is obvious but hang with me for a while ok?)between them they will almost certainly stop the wheel.  This is not good! If the riders arms are stiff the front wheel won't be able to move up over the rock. So any remaining momentum in your body will move your balance forward  and over the bars you go in a neat arc. This can be very painful and off-putting (Duh!). The next time the rider approaches a similar obstacle they're more afraid (fair enough) this  makes them grip more tightly and brake harder. The result is they either crash again, or get off and start pushing.

A relaxed rider won't slow down quite as much, the combination of a little extra momentum, no front braking at the crucial moment and relaxed arms, allow the wheel to bump up over the rock and  onward with little effort.

So if you're going slowly, it's essential to let go of your brakes as you approach an obstacle. This may mean going just a little faster, but the result is much less painful. I'm not talking high speed here, but let's face it, anything slower than a brisk walk is difficult on a smooth flat trail. On a steep bumpy trail going really slow makes things very difficult indeed.

An exception to this is a very tight switchback turn. If a trials style hop is out of the question (it is for me!) you'll need to slow right down to allow the smallest turning circle. If it's very steep you'll also need to hang way off the back. This kind of stuff takes practice, track standing is a good way to improve your balance and  it's one of the few 'street tricks' I practice. (It's not that I don't like them, I'm just rubbish at them).