The (very small) wiki article says:
"The stimulus voltage stimulates nearby nerves, resulting in contraction of the pelvic muscles and ejaculation."
If that is correct, then the normal triggers for ejaculation in the CNS are being bypassed, but the nerve axons and maybe their endings near the electrode of being stimulated, which would result in neurotransmitter release and the muscle contraction. There is another
wikipedia article on ejaculation, which describes CNS areas that could be involved in the process, but it does not go into the anatomical details and mechanics of the process or where the force is developed.
Electrostimulation can stimulate different cells depending on location of the electrodes (how close to different cells), the size of the cells (bigger cells or axons are more strongly stimulated than smaller ones, existing membrane potential vs. membrane voltage needed to produce effect.
Depending on the anatomy, location of electrodes, etc. It might be that the muscles involved are being stimulated directly rather than the nerve endings. This would bypass all neural elements. Blocking neural transmission combined with electrostimulation could provide an answer to this.
I was working on a project once (late 1990's), trying to get
amphioxus (an interesting sub-vertebrate
chordate) to breed. We would collect them (2-3" long little sliver shaped animals), put them in small plastic cups and give them an AC shock which caused both the males and females to release gametes. It worked if the critters were ready to breed, but no one doing this at the time understood the details of how the stimulation worked. However, the whole animals were being shocked (in groups) so it seems likely that the largest cells (probably the muscles) were being driven by the electricity.