No bull, the real deal.
I sound for enhanced masturbation to have a more intense ejaculation, not to stretch my urethra or push my limits. Sounding and the results I get are the only form of sexual activity that I can do better solo than I can with my lover, even though that idea is appealing. Solo is better because of how quickly I can respond to the feedback I get doing it, and if my lover could read my mind, laying back and enjoying it would be a wonderful thing.
I recently did this after several months and paid attention to my responses. Adequately cleaned and lubed, I began completely flaccid. I gradually inserted the 4mm Dittel sound using very short strokes while rolling it between my fingers so it does not "hang up" at either end of the stroke. For the first two inches or so, there was little sensation I could feel, although there was a mild "erective" response.
There was a slight resistance at the "J-curve", which is more like a "(" when I am soft. Willing myself to relax in the same way I do preceding peeing along with feather light stroking outside and gently pulling up of the glans around the sound in the same short strokes as inside and then sliding and rolling the sound in another 5mm or so on each down stroke caused that resistance to lessen and the sound to "give" and slide in snugly but without effort. During this time the "erective" response increased on each stroke accompanied by increasing arousal.
When I reached what I think is the outer sphincter, erection and arousal (and I sometimes have ED) increased rapidly on a kind of "will of their own" even as I was trying to observe this with "scientific detachment". As I went perhaps another 15mm deeper with the same tiny strokes inside and out, I experienced a growing hot flash and urgent sensation of orgasmic inevitability. Each of the maybe 10 strokes I did before ejaculating was almost electric in sensation. My guess is that the tip of the sound had entered the prostate at this point. I stroked until the first contraction was about to start and extracted the sound fully by the third contraction of a pretty impressive orgasm and a better erection than I get in external masturbation.
There was only the slightest irritation later from this; a little warmth during peeing and only a slight "pinch" at the mouth of the urethra from the saltiness. That disappeared in a couple of hours.
I think penetrating the sphincter rapidly increases arousal and leads to orgasm on its own, but going a little farther into the prostate "tricks it" into feeding back that it is fully loaded and ready for ejaculation. If I stayed at the sphincter, the prostate would load itself anyway and ejaculation would happen, just delayed several seconds. Once you figure out your "system", you can play it like an instrument. As orgasm begins to come on, you can extract the sound out of the prostate and sphincter to stop its advance and use the sound to stay "at the top" just beyond the point when orgasm becomes automatic for as long as you can stand. The longer you do, the more your testicles will load the seminal vesicle and the more contractions you will have until the prostate will no longer pressurize. In effect, you will be putting off pleasure to get more in a minute or so.
This can be very invigorating if done carefully and moderately. The two stimuli appear to be opening the sphincter and "pressurizing" the prostate and a wider sound is not necessary to do this. What is important, after cleanliness is addressed, is to take your time and use proper lubricant. Never push the sound in beyond a little coaxing and pay close attention to even the slightest sensation of pain and learn how to eliminate it. As you get aroused, endorphins will invigorate you and mask pain down to a slight ache inside, which can mean you are causing damage and don't feel it. If it can't be completely painless, stop immediately.
Unlike typical hand masturbation, toys, etc., this takes almost no distracting, tiring physical effort. Movements are very subtle and the effects, done right, are quite dramatic. Just give your private parts the same loving touch (and more) that you give to those of your lover and this secret pleasure is something to look forward to as an occasional diversion.