Click on any step below and use the scroll-wheel to move through the animation.
More power into the kick
Position yourself for the kick
Rear wheel position before the kick
The rear hub is right above the highest point of the edge, the wheel will provide the maximum torque horizontally, for an optimum gap distance. There is no vertical reaction, all your energy goes into the horizontal move.
The rear hub is a bit beyond the edge on the gap side, the rear wheel rolls down and this drags the bike down while you try to tuck for the gap crossing. The downward reaction (red arrow) is lost energy and makes your jump ineficient.
The rear hub is behind the peak of the edge, the wheel can get stuck behind when you kick. The vertical reaction upwards (red arrow) will flick the rear wheel up (face-plant guaranteed in most cases, this is no fun!)
Plan a: best case scenario
In the optimum case (picture a), the rear hub is positioned at the vertical right above the edge, this ensures the most kick back from the edge with a reaction force that is completely horizontal. From the kick alone, there is no vertical reaction and you will perform the longest jump.
Plan b: less than optimum
If you kick in the pedal while the hub is already standing beyond the highest point (picture b), then the reaction to the wheel torque (the force that reacts from your kick into the wheel) can be decomposed into a horizontal (green arrow) and a vertical (red arrow) components. In this case, the vertical component represents energy that is wasted downwards, that could have been used instead to propel yourself across the gap.
Instead, it pushes your rear wheel down and makes it more difficult to lift the bike up. Now, this would be the right approach for drop-gaps. You can play on this roll-down effect to kick your rear wheel directly towards your targeted landing spot if it is situated lower.
Plan c: definitely scary
If you still kick the pedal when the rear hub is positioned behind the highest point of the edge (picture c), the reaction to the wheel torque (blue arrow) will be decomposed into a vertical force upward (red arrow) and a horizontal reaction (green arrow).
That would be ok if you kept your weight behind or above the rear hub, maybe for a small re-adjustment hop to get right on top of the crest. But if you kick too hard and lurch forward over a gap, the upward reaction will flick the rear wheel up and tilt the bike down as you shift your weight forward.
This is a sure way to plunge over the bars. Effectively, it is as if the rear wheel was stuck behind a small step, and that could be the start of a face-plant.
Adjustment and pre-load hop when lowering the front wheel
Typically, when you are in perfect balance on the back wheel, on a rail or a pointy edge, the front wheel is fairly high up. Now, when you lower the front wheel before the kick, this rolls the rear wheel down by about a foot (with the hub coming behind the edge).
That's why in most cases, the compression hop before the kick is also a repositioning hop, bringing the rear wheel back again in a good position right before the kick.
If you want to avoid a re-adjustment hop, then start in balance with the rear hub just a bit behind the edge, so that when you lower the front wheel you end up with the hub exactly at the vertical to the edge (picture a), ready for the perfect kick. Try to focus on this and you'll notice a huge difference in your kicks.