Here's the problem: If you don't get your weight off the front wheel, it will slam into the rock wall. Pinch flat, bent rim, fall off the bike. But then your weight has to be off the rear wheel, or its hit the edge hard, with the full mass of your butt driving the tire into the rock. Pinch flat, sudden stop, ding rim, fall off the bike.
The key is a precise timing: Weight back, then forward, while the arms create a pull-push motion of the handlebars. As you approach the ledge, rise up, weight back, then pull the front wheel up onto the ledge. Next, push down on the front wheel, body up and forward. Last, tip your shoulders down to neutralize gravity and "lever" the rear wheel up. Sounds complicated. Let's watch a ledge attack and break down the moves. The following photos represent around one second of riding time.
The rollover works for smaller drops, up to about 18 inches. Make sure you have enough speed so the front tire won't "hang" by dropping straight over the edge. As you reach the drop, take the weight off your hands, transferring everything to your feet. Get your fingers OFF the front brake, completely away from the lever. Feel a bit of pressure on the UNDERSIDE of your fingers; a slight upward pull on the handlebars. As the front tire touches the edge, let off the rear brake too. Let your elbows extend and keep your body upright as the bike rotates down under you. (Your body stays in the same position while the bike tips, so the saddle comes forward between your thighs.)
Let off the rear brake before the front wheel drops down. (You don't want the bike to travel slower than your body -- if it does, you'll fall forward on the handlebars and endo.) Stay upright and keep your weight OFF the handlebars. As the rear tire rolls off the drop, let the bike rotate back to its former position relative to your body.
The Bunny Hop Jumpoff -- high-speed
For the bunny hop ledge drop, you launch the whole bike at once. To bunny hop off the edge, you need some real speed! At intermediate speeds, you're better doing a ramp-up drop (below). If you can't fly into the drop (or if there isn't room to land and control at a fast speed), you need to do the wheelie drop -- see below.
The goal is to have the rear tire hit a microsecond before the front tire, with all your weight on your feet. (If you land with weight on your hands, you'll dig in and lose it.) For the average bunny hop ledge drop, you'll do well if the rear tire hits while the front wheel is still about 6 inches off the ground.
The Ramp-Up -- intermediate speed
The simplest way to take a ledge is at intermediate speed. To do a ramp-up jump, you don't make any violent handlebar action and you don't compress the bike. You just make sure the front wheel continues straight out into space as you come off the edge by transferring your weight back and "helping" the handlebars stay up.
As you approach the edge, get your feet level and comfortable on the pedals. Take ALL the weight off your hands. Envision your front wheel just continuing into the space in front of the ledge, as though there were solid earth there. As the front wheel rolls onto the edge, push your feet forward, so it sends your butt back a bit. With your hands, you bring the handlebars forward and slightly up.
When done correctly, this ledge drop should be practically effortless. If you're putting violent muscle effort into it, or if you're not totally comfortable and in control when you land, you're not doing it right.
Hucking -- intermediate to low speed
While "hucking" can be a general term for launching off objects, it also refers to a specific technique for launching horizontally off a ledge. When hucking, you're throwing the bike forward off the ledge. This technique works at intermediate to low speeds. The lower the speed, the more pronounced the action required to launch the bike.
As you approach the lip of the ledge, stand tall and lean your body forward. Your weight is divided between hands and feet. When the front tire is just reaching the edge of the drop-off, lean forward even more. Your nose should be almost straight above the lip of the ledge. As the underside of the front wheel rolls onto the launch point, suddenly take the weight off your hands and throw your body up and back. Your feet push the bike forward as you re-center. Your shoulders should move back at least a foot or two in relation to the bike frame. As the bike lurches forward over the edge, fade your body down a little, letting your knees bend to unload the bike. Your hands push the handlebars forward in a straight line.. This action continues to "throw" the bike forward rapidly, so the front wheel doesn't start tipping forward until the rear wheel has cleared the edge.
This technique should bring the rear wheel over the edge of the drop-off with the bike oriented horizontally. As you land, your weight is already back. The front wheel is unloaded completely. Once you've got the technique behind hucking, it seems an easy and natural way to drop a ledge at lower speeds.
The Wheelie Drop -- low speed
The wheelie drop is used to head over a ledge where there isn't room to "fly" off. If you can't get up speed for a bunny hop or ramp-up jump, or if you need to land at low speed, the wheelie drop is what you need.
You need to be in a gear where you can put some power -- well, a LOT of power -- to the pedals. If necessary, upshift a couple of gears as you approach the edge. Time your pedal strokes so you're ready to deliver power with split-second timing.
Right as the front wheel touches the edge of the drop, rock your weight back over the rear tire and pull up on the handlebars. As you rock back, make a full pedal-stroke to propel the bike forward off the edge. Keep the bike horizontal, or even aiming a bit skyward, as the rear tire rolls off the ledge. If you don't get all the way off the ledge with a single leg-stroke, transition into a stroke with the other leg. (Practice holding the front wheel a tiny bit off the ground while you "wheelie" for a couple of strokes.)
If the bike rocks forward before the rear wheel leaves the edge, you didn't do it right. On a BIG drop, this can get you hurt. Do little ones until you're confident you can always keep the bike straight until it drops.
At very low speed, you may need to compress the front wheel down first, to get enough "front end air" to roll off the ledge smoothly. This makes the timing trickier, and makes it harder to control the tilt of the bike as you push forward. So for most wheelie drops, it's better to do a simple rock-back--pull-up--push-pedal, instead of the slam-handlebars-then-yank-up type of wheelie.
For a major drop, try to land with the rear tire hitting first. Ideally, the rear tire should be about 6 to 8 inches closer to the ground than the front tire. That way, your rear shock can absorb some of the landing before your legs need to react. Hitting rear-wheel-first gives you a split-second of "feel" that you're hitting ground, so you react in time.
The most common error in wheelie drops is "making your move too soon." If you power the pedal too soon, you may lose momentum and drop, nose forward, before your rear wheel has left the edge.