You can explore ‘La Petite Mort’ which is French for ‘The Little Death’. Think of it as an ‘Oh my God Orgasm’. It will leave your whole body twitching while you are struggling to breathe. The breathing issue is due to your diaphragm temporarily seizing and will pass shortly. The twitching may go on for several minutes and may start up again if your partner swats your butt.
Having one involves about an hour of continuous oral sex which, except for lesbians, might make finding a willing partner difficult. Lesbians have the advantage of being able to return the favor.
You can find some background info in the book ‘The Hite Report’ by Shere Hite. She describes the orgasm as a combined internal vaginal/anal and an external clitoral phenomenon. She found that difficulty achieving one is largely due to clitoral orgasms interfering with the combined orgasm.
To desensitize the clit I’ve found a three twenty minute stage process that works. Stage 1 involves servicing the clit continuously for as many orgasms that occur. Stage 2 Adds two fingers in the vagina to servicing the clit with one’s tongue. Stage 3 Move one finger from the vagina into the anus and service all three.
After an initial orgasm it may seem that nothing is happening. Keep pleasuring her while the internal orgasm builds. Once it starts up she may begin lifting her body from the bed while holding your head in both hands and grinding her pussy into your face. When the orgasm hits she will collapse twitching back on to the bed. Crawl up beside her and hold her while her whole body twitches uncontrollably.
The good news is that the two of you working at it should be able to get it down from an hour to about thirty minutes. If you can, suck her clit into your mouth and pleasure it incessantly before easing off when you go to finger **** her to another mind bending orgasm.