Gee, the biggest problem doing a Jumpstart (or beach start) is the fear. You stand there in knee deep water, holding a rope, as a boat on the other end of the rope starts to speed away.
In actuality, a Jumpstart is pretty safe, if you are of average strength and weight.
If you pull in too much rope, the boat will be going too fast when the rope tightens and it pulls the handle out of your grasp. Of course, if the length of rope you pull in it too little, a faceplant can happen.
Jumpstart is a nice term, but a Jumpstart is really more like stepping up onto the surface of the water with your ski, as the rope tightens.
So, stand in knee deep water, on a downward slope of the bottom. Pull in two arm lengths of rope (more or less dependent on the acceleration of the boat), yell hit it and drop the extra rope. Now two hands on the handle, arms pulled in and lean way back. As the rope tightens, make sure the ski tip is at the water surface, and you should be pulled up and onto the water. I drag my back foot as a sort of rudder to help keep me and the ski straight.
The prerequisite to do a successful Jumpstart is that you can ski fairly well on slalom. A lot of times, the Jumpstart will be like the last moment before you complete a deep water start on slalom, that is, the tail will sink a bit and may even drag in the mud.