well, forget about the whole idea of jumping into a shaka, it's not really a jump,you have to be using the sail to get your air,therefore you need slam back the sail as you carve- sheet in hard with your backhand and fully straighten your front one- follow the movement by getting your complete bodyweight over the rig, remember a shaka is a constant progress from looking gay to falling on to the rig to start to float to shuve-it, and than at some point to the actual shaka! another thing to remeber your body needs to keep moving forwards in the direction you are sailing in same as in a flaka you not really doing the move into wind!!you only turn to windward!(example:if you are on port tack-your board is going left,your sail right and your body pretty much inbetween!!) to be fair if it would be that easy it wouldn't be that interesting, but it's defenetly one of the best feeling freestyle moves!!
hope that helps!!
Marco