I'm not completely convinced. If the tube diameter is significantly larger than your girth, by not having the insert in, you are actually allowing MORE skin to be sucked into the tube. Also, once your girth is pressed against the insert, it would no longer be in contact with water and would not have the vacuum pressure exerting on it any further. The pressure would then be relatively higher on the exposed tissue which in its efforts to expand would draw blood away from base and subsequently replaced by blood inside body. And so I don't see how the insert would cause enough compression on base to have a restriction of blood flow.
I thought I read early on in these forums that you want a tube that is relatively close to your widest girth so you don't develop a deformed penis. I definitely don't want a cone penis. I'd rather have more like a baseball bat lol.
No worries. Doubts provide clarity in the pursuant of the knowledge. Let's break things down:
The insert reduces the total diameter for the BM. This is the "small" diameter you're looking for. Coming into the cylinder fresh, your skin is still nice and tight and not as loose. The expansion of your penile shaft is not as larger, where no restriction or constriction occurs at the moment. Using the comfort sleeve will reduce the chance for the skin at the base of the base from being sucked up into the wide diameter of the BM.
However, the moment after the first pumping session, within 5 minutes, your skin along the penile shaft and the base of the penis pulled to the point where you have 125% to 140% elasticity. Your nerves and blood vessels are less than 4mm below the skin. When your skin expands, your internal tissues also expand. This pushes the nerves and blood vessels closer to the outer skin, with a higher likely chance of being pressurized against. By using the BM's insert, now, you're compressing the nerve, the branches, and blood vessels between the internal tissues expansion and the outter skin. The best indicator for this is the numbing feeling on the second and third pump sets. Why that dull feeling of numbness? Because nerve and the branches are being compressed. Just imagine the blood vessels. With that in mind, let's go to the next point.
You indicated that the expanding girth presses against the insert and has no further expansion influence from the water/air under vacuum pressure. That is incorrect. The expansion for the surface area against the insert will stop, and the vacuum pressure continues to move forward towards the exposed area above the insert. You indicated that that well. However, the problem is the compression against the insert cuts off the nerve neuro-signals and the blood vessels. This is venous constriction at the highest. There is no more blood flowing into the penis at an optimal level. You can continue to force and force and force the blood in, but you're only introducing a small amount compared to a more loosely opening at the bottom. Whatever fluid is already in the penis, the vacuum pressure causes the balooning towards the overpressurized zone near the glans. The best comparison to this an innertube with a hole in the tire. Same amount of air volume, under pressurization of weight, the innertube stretches through the opening of the damaged tire until the possible innertube explosion. In our case, the penis suffers blisters, edemas, and blood spots/welps.
In the case of having a cone penis, where the bottom is larger than the top, the same can be said for a bat penis, where the base of the penis is small than the mid to upper part of the penis from over using the insert. This is why the cylinders come into play. Notice our brothers don't just have the BM, but also the MityVac cylinders as well. I have 2 cylinders and a BM. Funny isn't it? BM is awesome for compactness and on-the-go needs. BUT the cylinders are needed to keep everything, and I mean everything uniform. I have a 9in x 2in cylinder, a 11in x 2in cylinder, and another 11 x 2.25in cylinder on the way because I discovered my girth is expanding way too fast and it cuts off my blood after my 7th set of pumping. At this point, my fatigue kicks in, and I can't force any more blood in even at 70% erection.
PE is a crazy endeavor.