The yab-yum images (literally, Honorable Father-Honorable Mother) portray -- in symbolic terms -- various enstatic states. Yum, the Mother, represents prajna, supreme wisdom, which is also sunya, The Void, nothingness. Yab, the Father, represents upaya, the method (the particular, fleeting existences), which is also karuna, or the dynamic of compassion. When the two become one, the highest state is reached. Upaya is bondage without prajna. And Prajna, too, is bondage without Upaya.
In the less esoteric Buddhist sects, the Tantric rituals involving the adept's experiments to reach the enstatic realization of the union of yab and yum in his person is purely metaphorical. In the more esoteric ones, the instructions are taken literally as laboratory and consecrated use of the sexual sensations. The latter is either done alone (with a symbolic partner) or together with a real partner. What is often disregarded by the outsider is that the "congress of the holy" -- if we may call it that -- is precisely performed to negate sexuality: the mind and the senses are brought to the highest "madness, mind-blowing" stage and held there: orgasm is not allowed to occur.
The Secondary Anatomy
In yoga, which forms a principal element of Tantrism (Hindu or Buddhist), there is a theoretical secondary anatomy in every man. This abstract somatic system has centers, circles or lotuses located along an abstract spinal column. This anatomical model is purely an aid to meditation and is not supposed to correspond to the adept's biological body. That is the reason differing descriptions of this secondary body exist in various yogic books. And a teacher is most likely to describe this secondary somatic system one way to a particular neophyte and another way to someone else, because he knows that people respond in different ways to various psychological stimuli.
Three ducts are supposed to pass through the spinal column. In animals and ordinary humans (meaning to say, Tantric uninitiates) the central duct is closed. What opens it is correct meditation. Thus opened, the duct will then allow a mystic power -- the Hindu Tantrics call it kundalini (the coiled one), the Buddhist call it avadhuti or in Tibetan kun dar ma (the purified one) -- to rise from its sanctuary, which is placed between the anal and genital regions and is supposed to be the lowest of the centers.
On its ascent, this mystic power pierces the other centers along the spine until it reaches and fuses with the highest center, which is in the brain.
The other two ducts function in all living beings. But the Tantric adept tries to put these ducts under control by breathing and other exercises. Enstasy is reached by immobilizing the three ducts. In some Tantric Buddhist sects it is stated quite clinically (although as usual code terms are used) that control of mind, breath and sperm leads to enstasy.
Breath control, the easiest to achieve, presupposes ability to control involuntary physical processes. Seminal fluid control is tantamount to control of passions and desires. Mind control is the supreme achievement through which one ceases to perceive oneself as different or separate from absolute reality: The Void.
The Tantric Buddhists refer to the state of enstasy attained through this threefold control as mahasukha, great bliss, which is their most complete nirvana. Far from making the Tantric Buddhist a champion of eroticism, the yab-yum images in fact make him, as the Buddha before him, a conqueror of the mundane.