Downhill:
A few simple rules may apply when riding downhill. You will want to keep your weight back on the bike, and maybe even off the end of the saddle. Keep a firm grip on the handlebars, and your elbows low. Get a solid stance on your pedals, and a good grip on them with your shoes. Keep your pedals and crank arms even and parallel to the ground (attack position). Get off the saddle and crouch down for low centre of gravity. Scan the terrain well ahead, and be prepared for difficult areas in advance. Don't ride faster than you can handle, and slow down periodically by using the brakes a bit - control your speed by feathering your brakes ABS style.
Some challenging areas are best handled with some speed. Be careful of applying the front brake too much and either locking it up or skidding the front wheel if you get into a bind. Use your rear brake mainly, but do not allow the rear wheel to skid - incorporate the front brake as needed but do NOT allow the front wheel to skid!! If you must go down, go off the back or side of the bike, and not the front!! Protective gear (gloves, full helmet, knee pads, elbow pads, shin guards, etc..) are a good idea for the serious downhill rider, as well as a bike specifically set-up for down hilling.
Going over rocks on a steep down-hill: 
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, guess what? You're gonna stop and mostly, unexpectedly, more than likely go over the handlebars! Ouch!!
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