In the 1890s, Charles Leland described techniques based on ritual sexual intercourse in his "Aradia"
"And thus shall it be done: all shall sit down to the supper all naked, men and women, and the feast over, they shall dance, sing, make music, and then love in the darkness, with all the lights extinguished; for it is the Spirit of Diana who extinguishes them, and so they will dance and make music in her praise."
This idea, the Heiros Gamos or sacred marriage, is very old indeed. The ritual marriage of the Goddess Inanna to the shepherd king Dumuzi in the hieros gamos was the high point of the Sumerian sacred cycle:
"As for me, Inanna,
Who will plough my vulva?
Who will plough my high field?
Who will plough my wet ground?"
"Great Lady, the king will plough your vulva.
I Dumuzi the King, will plough your vulva."
"Then plough my vulva, man of my heart!
Plough my vulva!"
Startlingly similar to, and possibly the source of, the love feast of Aradia, are desriptions of the Tantric sacred orgy. This was performed at night in a circle of couples was controlled by the Lord of the Circle and his naked wife. The women removed their bodices and with them their social identity, becoming shaktis. The men picked a bodice at random from the centre of the circle and copulated with the woman to whom it belonged. The collective union of the night was made more orgiastic through the use of wine and hallucinogens. The darkness promoted visions and the setting, wine and hallucinogens combine to provide a set of behavioural triggers. Thus, the effects of divine union and the absorbtion of sexual energy are achieved more easily with repeated use.
Turn aside from the Shakti
who is present before the feast
She belongs to the Lord of the Circle
The conventional view of Tantra propounded by western anthropologists is that it takes the sex act for what it is: the foundation of life. The male Tantrika replaces sexual enjoyment with the absorbtion of the sexual energy of the female. The female Shakti generates the energy by "the vigour and skill with which she pursues the divine union": she reserves the physical pleasure for herself. The cosmic ecstasy produces a death and resurrection cycle leading to supreme knowledge.
The truth, though, is probably much more complex. Certainly, early western scholars confused Tantric eroticism with immorality - "lust, mummery and black magic". The breaking of taboo is, however, limited to the context of rituals such as kulaprakiya, where it represents liberation from duality (though this does not exclude sexuality or pleasure). Even the conventional representation of the yoni and lingam as penis and vagina or male and female deities is an over-simplification: they also represent, for example, sacred space and formlessness.
Geoffrey Ashe wrote (in The Finger and the Moon) "The secret is that there is no secret".
The late Gerald Suster commented (at a Talking Stick open forum) that you could print the innermost secrets on the front page of a newspaper and only those who were ready would understand them
Most commentators suggest that personal instruction is important to understanding: certainly this is one principle behind the wiccan initiation system. It's not that physical intercourse is necessary to pass on the techniques, simply that it may be easier and safer to learn certain things from personal question and answer interaction. Learning from others' mistakes is normally less expensive than learning from your own.
Magic has been defined as the alteration of consciousness or matter in accordance with will. So intention is the primary requirement in sex magic as with all magic. An intention so vivid that it can be visualised, felt, tasted and held throughout the working. Other than that, the requirement is for a method of raising and focusing the power of brain and will: sex is one of the most potent ways of doing this. For most practitioners, the attainment of the desired intention, rather than being an end in itself, is a means to an end, that end being "to obtain the divine ecstasy, and so attain knowledge and communion with the Divine Goddess." I've constructed a set of pages which indicate what's involved - the Flowing Well sequence. There are many approaches and mine is based around gentle visualisation, so if this type of thing isn't for you, find a good search engine and look for something more to your taste.
As with all magic, two good rules are never to summon anything which you can't banish and to be very careful what you wish for, since you may well get it.
It is arguable whether the remainder of the wiccan Great Rite is simply, as Gardner suggested "our system of props". It is certainly not necessary, but the repetition of familiar operations in a familiar environment certainly builds a set of learned responses which would be fully understood by most behavioural psychologists. Ritual bathing provides a break in time, fragrances added to the bath or washing water lend a sense that the occasion is special. The mind and body body learn to associate them with what is to follow. A circle or ritual area provides a break in space. Points of light gleaming amid the surrounding dark are not merely stage effects. They start the suggestions.
Other mechanisms may be used to initiate a slightly altered state of consciousness. There is much debate about the common wiccan prohibition of alcohol and drugs for a period before entering a circle. In the case of sex magic I tend mostly to agree. A glass of wine relaxes, more, in a working which is specifically intended to affect the brain, might in extremis cause something which will fry the psyche utterly. Constrained breathing exercises, perhaps visualising light flowing with the breath, normally work at least as well.
Having built up a good head of 'out-of-ths-world-ness', the participant(s) now perform a consensual sexual act designed to maximise pleasure in the context of the working. Whilst they do this, they aim to maintain with clarity the intended result. Not to visualise it but to experience it for real. Eastern sources advocate an approach where one partner of a couple does most of the work whilst the other concentrates on the visualisation. Western exponents normally aim for a balanced approach with partners taking equal roles. The Eastern approach is easier. Most tutors suggest that couples practice separately as well as together. Setting aside time for a solo ritual which may involve masturbation may seem strange, but "to us it is natural..."
Grounding is the term normally used for returning to what is perceived as normal. Allowing the brain time to adjust is important. The banishing or dismantling of the circle, erasing the sacred space, allows such time. Food, wine, salt, coffee, stamping one's foot, hugging people are all ways of affirming contact with the everyday. The communal cakes and wine of a wiccan ceremony do this very well. In the Dennis Wheatley universe, failure to properly banish has dire consequences on the astral. It most certainly does on this plane. Violent headaches, shakes, head feeling full of cotton wool, inability to concentrate, irrational fear. If that sounds like something from Monty Python, the reality can be worse, but you have to have been there to understand. Never forget that the Great Rite is intended to do things to your brain. And never forget that it is meant to work.
It takes practice to obtain results - the hard part is convincing the brain to accept that it will work. But, once it starts to "work", even if the results are bad ones like the list above, the brain gradually adjusts and the results become more predictable and reliable.