The key to down hilling is relaxing your upper body. The steeper and rockier the hill is, the more tightly the rider tends to put a death grip on the handle bars. Most riders tend to slow down as they approach obstacles such as rocks, then apply both brakes.
If you don't apply your brakes, the rock will stop your wheel. This isn't good, as the rock can throw you off balance and completely kill any type of momentum you have.
Relaxed riders won't slow down as much. The combination of extra momentum, no front braking at crucial moments will allow the wheel to bump over the rock and continue onward with little effort.
If you are going slow, it's essential to release your brakes as much as possible when you approach an obstacle. This may entail going a bit faster, although the result is much less painful. On steep hills, going really slow will always make things much more difficult.
One exception to this is a very tight turn. If a hop is out of the question, you'll need to slow down to allow the smallest radius of turning circle. This kind of thing takes practice, although track standing isa great way to improve on your balance.
Although down hilling is one of the most extreme methods of mountain biking, it can also be one of the most dangerous. If you're new to mountain biking you shouldn't start out with down hilling, as it takes a lot of practice.
With a bit of practice and knowing the right techniques, technical down hilling is something you'll find fun. It can provide quite a rush and a lot of excitement for those who seek adventure.