i could picture a longitudinal wave inside the water gap, a push pull wave traveling though the aligned water molecules, Stan says the voltage never drops to zero in between the pulses, and the water maintains a voltage, this means the water stays aligned in the water gap, so with each pulse, it is an attraction force pulling on the water, then pulse off, it lets go, so it is a longitudinal wave of electrical stress.
its like if you have a rubber band and you keep pulling directly outwards on the ends, back and forth, you are creating a compression/expansion wave in the longitudinal direction. same thing happens on the water molecules.