When descending a steep pitch, you need to move back off the saddle. The steeper the descent, the further back you need to move your body. The goal is to place your center of gravity directly above the spot of ground between your tires. For a really steep spot, you may even need to drag your butt on the rear tire, while your chest touches the back of your seat.Body behind seat, feet 90° to gravity, knees flexed, eyes forward, rear brake hard on, front brake feathered. Problem? Look at the arms. The elbows are locked straight, so this rider isn't able to let the body stay upright to gravity (moving independent of the bike) when the bike rocks over obstacles, and isn't able to steer because both arms are locked straight.
If your position is right, you should be neither "hanging" by your hands (weight too far back), nor leaning on your hands (weight too far forward). You should have your weight equally on each foot.Notice the feet -- they should be level. Extending one leg completely may feel more comfortable, but all your weight is on one side, messing up the balance. And when you reach the flat, you won't be able to get your crotch up over the back of the seat -- so you'll go down.
Grip the rear brake. Maintain a moderate breaking force as you enter the steep. Feather (lightly pump) the front brake to control the descent. If you put too much "grab" on the front brake, a small bump may make you endo. If the rear tire starts to skid, back off slightly on the rear brake and pull back a bit on the handlebars as you push the pedals forward slightly. (It's the same "feel" as if you were rowing a boat while standing in a crouch.) Don't brake hard on the tricky sections -- this is where you "roll it." Do most of your braking immediately before, and immediately after, a hairy spot.
Keep the bike moving -- slow enough that your momentum doesn't throw you out of control down the slope, but fast enough to keep your balance and bang through the rough stuff. It can be hard for newbies to get this concept, but on steeps the rule is: easy spot = brake; tricky spot = go faster!Feet level to gravity, knees flexed, body behind seat, arms relaxed with elbows slightly bent and angled out, eyes forward. Now we play with the brakes!