Saturday, November 27, 2010

Under-thrusting, an explanation

A vintage Kirby Morgan Mk 16

Foreword: This under-thrust explanation is being written in reply to a comment posted by Eric on my entry for November 24, 2010 entitled “Under-thrust training” in which he wrote: “I've never understood how someone could thrust around it. Of course, I would think something like that would have to be a fluke or someone would have to have a lot of skill. However, you might've mentioned the possibility of poorly-fit diaphragms.” An explanation requires some detail so I want to post it as a blog entry rather than a comment so more readers will see it.

Under-thrusting defined: What I’m talking about is the intentional under-thrusting (forcing the tip of the penis under the rim) of a diaphragm worn for contraception and/or upper reproductive tract protection. Under-thrusting of a contraceptive diaphragm is far more likely during vanilla sex if the diaphragm is too small, but it is fairly rare if the diaphragm is properly sized and correctly inserted because the male partner is almost always interested in giving his partner her pleasure first or just getting off himself and most guys I’ve been with are trying to reach my cervix and none of those goals are easily accomplished by acute angle thrusting of the male penis which is necessary for successful under-thrusting.

Arousal: There are two factors that will determine the ease with which a correctly sized D can be under-thrust. First is the degree to which the woman is aroused. That matters because when fully aroused a woman’s vagina will lengthen (called tenting) by 5 cm or more (depending on the woman) as the uterus and cervix are pulled up and back out of the way of a thrusting penis. That means that the anterior rim of the diaphragm will be pulled out of the wearer’s pubic notch at least to some extent so there may be a gap between her pubic bone and the anterior rim of her diaphragm.

Missionary position: A few minutes after correct insertion the dome of the D should have developed a seal against the anterior wall of the vagina so the rim should not drop down unless the seal is broken, but if the woman is on her back and penetrated in missionary, if her partner thrusts up at an acute angle (into her anterior vaginal wall) when she is fully aroused he may be able to push his way between her pubic bone and the rim and thrust beneath it. Then he is inside the dome and if he ejaculates sperm is placed against her cervix. That’s why spermicide is used as a back-up.

From behind: When the woman is taken from behind while bent over so she is penetrated doggie style, the gap (after tenting) between the pubic bone and the rim of the diaphragm is even larger. That’s because the uterus and cervix are pulled deeper still by gravity when the woman is on all fours or standing bent over with her hands on her knees. With his partner in that position the man has to thrust downward at an acute angle if he is going to try under-thrusting his partner’s diaphragm. Even with a correctly sized diaphragm and during vanilla sex because the rim can be pulled so far out of the woman’s pubic notch it’s in this position that there are the most complaints from men about hitting the rim, which is uncomfortable for most of them.

Rim Style: The second factor that determines the ease with which a diaphragm can be under-thrust is the type of rim (coil, Arcing or flat spring) of the diaphragm the woman is wearing. There are practical reasons for a woman to wear an arcing spring or a coil spring rim, however both the arcing and coil spring rims are easy to distort when thrust into at an oblique angle without much indication to the woman that it is happening. That is not the case with a flat spring rim.

The only way a flat spring rim will compress is when it is compressed in the plane of the rim, thrust into edge on not at an angle to the plane of the rim. Therefore, when the rim is hit at an oblique angle as it almost always is the rim will not compress and it’s like hitting a metal wall. The penis will drive the uncompressed rim painfully into the woman’s vaginal walls alerting her that her partner is doing or trying to do something he shouldn’t. And ramming a flat spring rim that way will be painful for both partners.

The arcing spring rim is the most popular with doctors because it can be worn by women with a wide range of vaginal muscle tone and it is easier to insert correctly. However if the woman needs a menstrual gas guard that can be used effectively during dives deeper than 30 feet and one that is difficult to intentionally under-thrust then the flat spring Reflexions is the only one that meets those criteria. There are a few men in Vegas who are masochists and sadists and these are the guys who will enjoy the pain of trying to under-thrust a flat spring diaphragm and enjoy hurting his partner and perhaps impregnating her. But if they try she will certainly know what he is doing.

Tradeoffs: Ideally, except for when a woman is on her period a cervical cap would be the best barrier to wear for dive-sex as it can’t be under-thrust and if correctly fitted it is very difficult to dislodge with a thrusting penis. The problem is that for petite women or women in their teens their pelvic anatomies may be too small to be fitted with FemCap (the smallest is 22 mm) and quite often their partners are hurt by hitting the removal strap or if that has been trimmed away the dome and find it uncomfortable. So a latex Reflexions flat spring diaphragm is probably the best all around gas guard available for petite or very young women.

Sex in a Kirby Morgan

In the Pit: On Thanksgiving Day while my other guests were watching football I took Adolph to the bottom of my 68 ft deep pool, the pit. He isn’t interested in American football and he wanted to see what it would be like if we both wore surface supplied Kirby-Morgan 47 helmets. I was CD7 and we both enjoyed ourselves. He is something of a sadist and I’m a masochist so when he battered my cervix through the dome of my Reflexions he actually hurt me enough to bring on a cervical orgasm which was awesome! He was bouncing me around pretty severely, but with chin strap, fitted head cushion, neck pad and the neck dam/neck ring assembly and twin-pin locking collar to hold the helmet securely in place I was quite safe, not in nearly the danger I would have been in if I had been wearing a KMH 16 [shown in the image at the top of this entry] where the latching assembly could be unexpectedly broached and have the helmet separate from the yoke assembly. I wouldn’t say the KM 16 is unsafe, but the latching mechanism that holds the helmet to the yoke and neck dam is an obsolete design more than 30 years old.

The KM 47: Adolph and I helped dress each other in our KM 47s so I got to pay close attention to how it should be done. It was a bit of a struggle though we put on the helmets standing on the shallow ledge so we could just submerge after we locked ourselves in to get rid of having the heavy helmet on our shoulders as quickly as possible. If I’m being thrown around during an intense sexual encounter I’m a lot happier with the much more secure latching assembly of the KM 47 holding the helmet sealed. I love the way the head cushion snaps into the helmet shell and can be custom fitted to make sure I could get my long hair (in French braids) comfortably and safely contained in the helmet. With everything fastened properly (including the chin strap) I felt snug and safe in my KM 47. The helmet’s weight is about 33.5 lbs and mine has been adjusted with weights for balance and neutral buoyancy in fresh water. A drawback of the KM 47 (or any KM mask or helmet) is that there is limited visibility through the polycarbonate lens when compared with my Ocean Reef Space Raptor FFM and because of the helmet’s size there is inertia as the diver turns her head.

But the reduction is visibility was offset in my case by the head protection the fiberglass and carbon fiber shell and head cushion provided when Adolph repeatedly shoved me into the wall during our encounter. When I tried to stay against the wall he would pull me away and then shove me back so the back of my helmet was repeatedly slammed into the pool wall as he thrust into me. I wouldn’t have escaped w/o a concussion if I had been wearing only a neoprene hood and FFM. But that’s just the way Adolph is. He extremely strong and goes after sex with a woman with amazing intensity. He knew better than to try and under thrust my gas guard though it is one of his fave tricks with most of his other partners. I would say he was brutal if I hadn’t enjoyed our encounter so much.


  1. Thanks for the explanation. Like I said before, I've never the experience of taking anyone with a diaphram (being I only had one sexual relationship, and part of me wishes things would have worked out better with the two of us; just feeling a bit lonely right now), so I never understood the underthrusting idea.

    Do you happen to have a picture of that KM 47 or a link for one? I think you had something on your Ocean Reef mask. If I were to have some submarine fun with you, I'd probably want to be surface supplied with a FFM. That way, we'd only have to worry about the hoses rather than the tanks.

  2. Hi Eric, here is a link to the KM 47 on the Kirby Morgan site:

    Under water sex w/o tanks is very liberating. You can surface supply an Ocean Reef FFM too. You just need to be sure you have plenty of hose or you can jerk the mask and run the risk of flooding it. The KM is excellent for head protection if a girl is going to be knocked around. After an Ocean Reef a KM can be a bit claustrophobic, but the KM 47 is a very safe helmet for an intense sexual encounter.

  3. "The KM is excellent for head protection if a girl is going to be knocked around."

    Well, I wouldn't anticipate being that rough. Though, I do like to try different things. Some of the things you do are very intersting to me, and I hope to find someone who'd be as curious/adventuresome.

  4. Now that's my kind of mermaid :)


  5. Great to see your having fun in the Kirby Morgan gear. Have you ever tried the Kirby Morgan M-48 SuperMask? It has a removable lower Pod, that could be fun too.

  6. Hi Anon. I haven't dived the M-48 super mask yet, but it does look like fun!


Blog Archive

Lijit Search



About Me

My photo
Powys , Wales, United Kingdom
I'm a classically trained dancer and SAB grad. A Dance Captain and go-to girl overseeing high-roller entertainment for a major casino/resort