Neat synchronization of pedaling and jump impulse
1° Start pulling on the bars, slightly lifting the front wheel with your bad-foot pedal (left if you are right foot).
2° Accelerate the move on your right foot pedal, with full extension while kicking on your good pedal.
3° The kick and jump extension must be synchro with the rear wheel leaving the edge of the gap.
You end up in full extension over the gap, pull up the bike as much as you can in front of you. When the rear wheel touches down, full brakes on (else, you'll fall on your back). Start from smallish kerbs to get the feel. See this move in a video
Finding the right synchro:
The first quarter turn with the good pedal is just to control the speed of approach of the gap, the second quarter turn on the goofy pedal is to lift the front wheel, keeping it horizontal in the air while folding back like a spring before extension. This brings you ready for an accelerated kick in your right pedal with extension for the jump. Before the pedal kick, try to keep the front wheel as low as possible on the horizontal, to have extra torque when giving the pedal kick (its like lowering the front wheel before a normal pedal hop).
Shrinking the run up distance
You must learn your distance marks, which depends on the gear ratio you use. Do this by moving the bike backward, from the final pedal position before the kick, to the initial pedal start position (just let the pedals' position tell you when you are there). After a while, you can play around by reducing the run up distance to a minimum, starting directly on your bad pedal (then body movement plays a bigger part). Practise on virtual gaps, between low kerbs to get your marks, before you launch this move on bigger stuff.
Then off course, you can launch this move to go up or down. Though, if you accelerate in a downward slope, you'll have to pull more on the bars to get a proper position in the air and avoid a nasty dive. It's all about practise, practise... and practise again.