I do not know if someone else has presented this yet, I don't remember who or what I read this from, but I think it may give an insight as to how this "water arc" can even move the piston. At first glance it should not be possible. Here it goes.
First of all, conventional science may well be right when they say "water is only an energy carrier". What if the water itself is not providing the motive force? What if it is only a mechanism to transfer the electrical energy supplied through the plug to the piston? That's exactly how I think it works. The water is only acting as a means to an end. So, if we were to crank up the amps going through the arc, we could turn up the force with which the water strikes the piston, thus giving us the ability to control the power output of the engine.
Sorry if everybody already figured this out or something. Just thought I'd post.