Downhill Switchbacks 101: Approaching the downhill switchback, aim for the high side of the trail. Let off the front brake. Keep the bike rolling! Begin the turn as the front tire passes the "middle" of the downhill path. While keeping your body straight and balanced, lean the bike in the direction of the turn (the bike tips relative to your body). Feel the saddle pushing against your downhill thigh.
Level your feet in the pedals. (Many riders try switchbacks with the outside leg fully extended. This is an advanced technique. When learning, you'll find balance is difficult with the outside leg straight, because you've committed all of your weight to one side of the bike.) Usually, control is best if the inside foot (downhill foot that will become the uphill foot after the turn) is forward.
Turn the handlebars downhill. As the bike turns to face down the fall line, let off the rear brake a bit. (The front brake is completely off.) Let the bike roll around the turn. Don't force it. Most of us have one direction that's easier (usually, if we get on the bike from the right side, we can turn most easily to the right). As the bike comes into the new direction of travel, "unlean" the bike.
Switchbacks 102: Once you can track around sharp downhill turns with the bike level, you're ready for the next step. Instead of leaning the frame downhill, then steering into the turn (as above), you'll "power" the turn with the downhill hand. While the downhill (inside) hand controls the sharpness of the turn, the uphill (outside) foot provides a counterweight to keep your center of gravity over the bike.
Slow to a safe speed as you approach the switchback, but you should be at walking speed or higher. Drift towards the uphill side of the trail. Put your inside (downhill) foot forward. Let off the front brake completely.
Just after your front tire passes the mid-point of the turn, put some weight on your downhill hand. Press the handlebar "down and away," with your hand moving slightly away from you towards the center point of the turn as it pushes the handlebar downward. This simultaneously leans the bike, turns the front wheel, and puts some extra "trail-grabbing" weight on those front treads (so you don't slide the front tire through the turn).
At the same time, press back and down on your uphill (outside) foot. The tighter and faster the turn, the more weight you put on the outside foot, and the further you press the pedal down. Don't let the downhill (inside) leg go loose -- you may need to power the inside pedal if you tip towards the inside as you complete the turn.
After passing the mid-point of the turn, smoothly let the pressure off of the handlebar. The weight you've been putting on the handlebar is now transferred to your feet. Begin to balance the weight between the outside and inside feet, letting the bike "unlean." Just as the bike is completing the turn, put power to the inside foot. This brings the bike back up fully vertical, and accelerates you away from the turn.
Advanced Downhill Switchbacks, Speed Turn: Advanced riders don't want to slow down and inch around a switchback. To ride a tight turn with speed, the bike must be tipped sharply into the turn. This requires raising the inside pedal fully up so it will clear the ground.
To make a speed turn on a switchback, approach on the high side of the trail. As the front tire reaches the middle of the downhill path, throw your weight onto the uphill pedal. Release BOTH brakes. Lean sharply into the turn.
As the bike tips, move the handlebars "in" towards the center of the turn. For a sharp lean, you'll put more of your weight on the uphill (outside) hand, so your weight stays centered. (This is different from Switchbacks 102 above.)
In the middle of the turn, use the handlebars to control the angle of the bike frame relative to your body. To keep the hold on the trail in the second half of the turn, some bikers "unlean" the bike, while maintaining a lean on the body. This puts the tire tread more vertical to the trail surface. As the bike pivots around, the body "meets up" with the bike at the completion of the turn. Don't apply any brakes during this maneuver, or the bike will go down.
Keep your weight on the outside foot until you've finished carving the turn. To straighten at the end of the turn, take the pressure on the handlebars and let the bike come upright.
Advanced Downhill Switchbacks, Nose Wheelie: Some switchback turns are too tight for the bike to roll around -- the distance from the uphill edge of the higher trail to the downhill edge of the lower trail is less than the length of the bike. The nose wheelie requires a bit of practice, and is for advanced riders only.
Slow as you approach the switchback on the high side of the trail. Keep tension on both brakes. Tip the bike sharply downhill, turning the handlebars to aim the bike for the inside corner of the turn. As the front wheel touches the downhill trail, simultaneously tip the bike outward (away from the direction you're turning) and hit the front brake hard. Put your weight equally on both hands.
As the rear end of the bike pops up into a nose wheelstand, let the body of the bike pivot outward. The rear wheel will continue in the direction you were formerly traveling, while the front wheel is frozen on the flat part of the downhill trail. As the bike rotates to the new direction of travel, let off the front brake and put your weight back to drop the rear tire. Immediately put power to the pedals as the rear tire lands.
Uphill Switchbacks: For an uphill switchback, you need to "throw your body" through the turn. Start on the downhill side of the trail. As you start the turn, throw your body upwards and towards the outside of the turn by pushing yourself uphill with your feet and hands. Timing is everything: get your body high before the bike hits the steep grade of the fall line.
With your body high, you're ready to power the bike up into the turn. Let the front wheel climb the switchback while you feel the weight with your feet. Keep your body forward! Once the front wheel has climbed to the high side of the switchback, push down hard on the handlebars. You're raising your center of gravity to the new (higher) path by transferring your weight to the handlebars. Put most of your weight on the outside hand -- the one that will be "uphill" when you finish the switchback.
Make a couple of good hard cranks as you feel the weight coming down through your arms -- almost as if you're doing a pushup on your bike. As the bike turns into the uphill path, let your body lower again as the bike comes up under you.