I have used a straight dittel, 4-5mm, starting completely flaccid. With a thin water-base lube like Astroglide, I was able to reach the prostate region effortlessly. There must be a slight bottleneck there, because it is easy to know I have reached it. The sensation is something below "feeling" and more like "just knowing".
"Fucking" that region with the sound, with an overall stroke of two inches rapidly brings on ejaculation. It's not exactly involuntary, but it certainly seems like cheating because only light outside stroking is needed and without any masturbation fantasy. It comes on quickly enough so that erection is still only starting. The intensity of contractions seem greater that "normal" sex.
I'm no expert on this, but there seems to be one pre-ejaculation contraction that I assume is to purge the tube of pee with fluid of correct pH. When that happens, I extract the sound to stay ahead of ejaculation. Since the sound holds the tube open, it can take two or three contractions to fill it, so I have the sound out at the end of the second. That gives a pleasurable slightly burning sensation that improves the experience.
If technology and technique were applied to doing this, I think it could be superior to e-stim because of how nerves function. In another post, I suggested a urethral stimulation device like this:
The device which would consist of a (not too) 4mm flexible rounded-tip tube connected to a pumping device that would alternately inflate (perhaps another 3mm overall) and then release the pressure as a form of speed controllable internal vibration/pressure. It would stimulate the prostate internally and could be combined with in/out motion or be inserted and used hands free.
Then I would train my nurse-lover to do it for me.............