I haven’t done it enough to really say what it is, the closest I can come up with is that it opens the floodgates leading into the penis; whether its muscle contraction or something else is hard to say. The feeling is so different that I haven’t had the time to really discover what it is for sure, other than it sure seems to work. Theory is probably right because hitting most nerves from the outside causes some sort of muscle reaction.
Thinking out loud now, but perhaps instead of a contraction its a complete relaxation of the muscles of the penis, which allows normal pressure to expand it, I know the penis is not a muscle but the valves for opening and closing would be. If these muscles are not in any way impeding the flow of blood in, and you can maximize the normal erection process beyond what it had really been designed to do. Remember that the normal erection is a planned event in the body, and what if this maximizes the total volume going in without regard for the regulated amount that would normally constitute a normal erection for that person. Basically fooling the body into doing way more than it was designed to do instead of using outside force to accomplish the same thing.
Whatever, I might just be thinking out of my ass but I still haven’t quite figured it out. thats the best I can come up with. This probably doesn’t answer your question at all unfortunately but I will pass on whatever I learn over the next little while. I have no problem passing on info but I’m still not quite getting it myself.
Hope it helps, but like I said I’m just kind of thinking out loud. I am sure that someone far more knowledgeable will come and correct me on this.