I appreciate all the replies.
I wish to comment on something. (Sorry for my lack of intellectual or should I say scholarly vigour at present. I have no more Internet or even computer at home anymore, and so have to do this from my local library, where I feel surrounded not just by other people (not that they are looking over my shoulder, but still...) but also where being immersed in wi-fi EMF fields is making me feel a bit dizzy right now).
At the moment of orgasm, the 'animal' aspect of our nature wants to push that energy outwards, downwards. To resist this urge and retain the semen in the body, drawing the energy inwards & upwards instead is actually reversing this usual course of events. It requires much self control, a certain measure of detachment and a lot of awareness of mind and body.
I agree with the member who said is needs to be done with the right instruction. There are a few masters around but one needs to be careful to find a properly qualified one. As this is a forum for the Buddha-Dhamma, which represents truths higher and more sublime than merely learning how to recirculate energy and thus live a longer and much healthier life, I hesitated to mention where I am getting this information from, but so long as no one thinks I am advertising, it is from Mantak Chia, a Taoist Master. He teaches this art in a completely non-religious way, by the way - he is actually a Christian by faith. It's just about energy. Anyone can learn to do it, religion doesn't come into it.
One thing I read was that one must not misuse the sexual power one can gain from this practice. It is intended to save the vital forces and recirculate them so that this energy can be put to good use, ultimately making one not just healthier but also a more loving partner to one's wife and a more helpful member of society. If one goes about having insubstantial relationships based upon sex alone, using women and abandoning them like a commodity, that is a misuse of the art, according to Chia. Maybe that is why it was traditionally not taught widely, but only to a few selected persons.
The truths of impermance and self-less-ness of all phenomena (in the sense of the upanishadic 'atman') are still much more earth-shattering than this, but I wanted to share how freeing it is to be letting go of the guilt and shame regarding sexual pleasure. I know it is impermanent and not-self. It is just feeling, it comes and goes. But somehow I feel as though a huge burden has been lifted from me. I am no longer at war with myself. Sex is not sinful, it is just a force of Nature. But one should know how to engage in it for one's benefit, rather than for one's harm and depletion. That is what Taoist sexual practices are for.
And regarding that warning someone gave about 'danger', as I said the only danger is if one misuses the powers that one can gain over time, having one-night stands etc (a moral danger). But that is caused by the person, not by the practice. There is no physical danger from retaining the seed. There can be some discomfort if one does it incorrectly, but not danger.
Then the Blessed One, picking up a tiny bit of dust with the tip of his fingernail, said to the monk, "There isn't even this much form...feeling...
perception...fabrications...consciousness that is constant, lasting, eternal, not subject to change, that will stay just as it is as long as eternity."