On flat water don??t think of it as a jump but a pop or bounce.
To do any of the new freestyle you only have to get the fin clear of the water.
The movement of the rig once the fin is clear automaticaly makes you got higher.
The pop or bounce is generated by having a straight front leg after you have depressed the tail. You spring off the front leg. Timing is the most important thing and you can practise this very easily by going at 90?? to the wind, bring your boby over the center of the board and just bounce.
Give it a try and I bet you will spend the rest of your time trying not to go high when you are learning freestyle because this is one of the problems when you are learning.