The tantras are the teachings and writings that set out the practices of Vajrayana Buddhism, the stream of Buddhism prevalent in Tibet. The tantric teachings are based on the principle of the transformation of impure vision into pure vision, through working with the body, energy, and mind. Tantric texts usually describe the mandala and meditation practices associated with a particular enlightened being or deity. Although they are called tantras, the Dzogchen tantras are a specific category of the Dzogchen teachings, which are not based on transformation but on self-liberation.  According to some sources, the word Tantra is made up by the joining of two Sanskrit words: tanoti (expansion) and rayati (liberation). In this sense, Tantra means the liberation of energy and expansion of consciousness beyond its gross form. In other words, Tantra is a method to expand the mind and liberate the dormant potential energy, and its principles form the basis of all authentic yogic practices.
Tantra is also not a religion. It is a non-denominational spiritual system that can be practiced by anyone, given the right attitude and dedication.
The tantric writings, teachings, and practices are purposely obscure so that only initiates – those who had demonstrated that they reached certain levels of spiritual achievement – could understand them. Traditionally the true tantric teachings, at least those that were authentic, were closely guarded and only transmitted orally from master to disciple after long periods of preparation, practice, and purification. Luckily, these spiritual gems are more widely accessible to modern spiritual seekers then in ancient times.
Tantric Sex, Sexual Yoga
Tantric sex or sexual yoga refers to a wide range of practices carried on in Hindu and Buddhist tantra to exercise sexuality in a ritualized or yogic context.
In the past Tantric sex may have been associated with antinomian or elements such as the consumption of alcohol, and the offerings of substances like meat to deities. Moreover, sexual fluids are viewed as “power substances” and may have been used ritualistically, either externally or internally.  While there may be some connection between these practices and the Kāmashāstra literature (which include the Kāmasūtra), the two practice traditions are separate methods with separate goals. As the British Indologist Geoffrey Samuel notes, while the kāmasāstra literature is about the pursuit of sexual pleasure (kāmā), sexual yoga practices are often aimed towards the quest for liberation (moksha). 
In his book “The Perfect Matrimony” Samuel Aun Weor elaborates the following about Sexual Yoga:
In India there are three types of tantrism; first, White Tantrism; second, black tantrism; third, grey tantrism. In White Tantrism Sexual Magic is practised without the spilling of semen. In black tantrism there is spilling of semen. In grey tantrism, one spills, and does not spill the semen. Sometimes they spill it; sometimes they do not. This type of tantrism leads the devotee to black tantrism. Within black tantrism we find the Bhöns and Dugpas of the “Red Cap”, terrible and perverse black magicians. These malevolent people have disgusting procedures to reabsorb the semen through the urethra after having miserably spilled it. The result is fatal because the semen, after having been spilled is charged with satanic atoms, which on re-entering the body acquires the power to awaken Kundalini negatively. It then descends to the Atomic Infernos of Man and becomes the tail of Satan. This is how the human being separates himself permanently from his Divine Being and sinks for ever into the abyss. Everyone who spills the Glass of Hermes is rightly recognised as a black magic In Hindustan, Sexual Magic is known as Maithuna. It is also known as Urdhvaretas yoga and those who practise it are called Urdhvaretas yogis. In all truly serious and responsible yoga schools, Sexual Magic is practised in a very secret form. When a yogi couple, man and woman have been well prepared, they are taken to a secret place where they are instructed about the Maithuna (Sexual Magic). The couples unite sexually to work in the Great Work under the su pervision of a Guru (Master). The man, seated on a rug on the floor in the Buddhic position, with his legs crossed in oriental style, en ters into sexual contact with the woman. She sits upon the legs of the man so that her legs encircle the male’s trunk. Clearly by sitting upon him in this way the phallus is then taken in. Thus the man and woman are sexually united. The yogi couple remains in this state for hours without spilling the semen. It is the duty of the yogi not to think during the practice of Sexual Magic. In those moments both man and woman are in a state of ecstasy. The couples are thus, deeply in Love. The creative energies rise victoriously through their respective channels to the chalice of the brain. Animal desire is rejected. Then the couples withdraw from the act without having spilled the semen. This way of practising Sexual Magic in the oriental style may be very uncomfortable for westerners. Nevertheless it is recommended for those people who are not able to avoid spilling the Glass of Hermes. With this practice Gnostics can train themselves sexually to learn to refrain, and avoid the spilling of semen. Gnostic couples do not need the physical supervision of a Master, but could however invoke the Masters of the astral so that they help them. (The couple must be alone.) It is necessary that during the practice of Sexual Magic there is no animal desire. Remember that desire is diabolic. The “I” is desire. The “I” is diabolic. Where there is desire there cannot be Love be cause Love and desire are incompatible. It is necessary to know that desire produces illusion. Whoever desires, believes himself to be in Love, feels himself to be in Love, could swear he is in Love. This is the illusion of desire. How many times have we seen couples who claim that they adore each other? After marriage the castle of cards falls down and what remains is the sad reality. Those who believed they were truly in Love, in reality hate each other, and once desire is satisfied, failure is inevitable. Then we only hear complaints and regrets, reproaches and tears. Where was Love? What happened to Love? It is impossible to Love when there is desire. Only those who have already incarnated their Soul really know how to Love. The “I” does not know how to Love. Only the Soul knows how to Love. Love has its own atmosphere, its flavour, its happiness. That is only known to he who has already killed animal desire. That is only known and experienced by he who has already incarnated his Soul. Love in no way resembles that which people call Love. What people believe to be Love, is only deceiving desire. Desire is an illusory substance that is combined astonishingly in the mind and in the heart to make us feel something that, though not Love, makes us firmly believe that it is Love. Only the horrible reality that appears after the consummation of the act and the satisfaction of desire shows clearly that we were the victims of illusion. We believed we were in Love, and really we were not. The human being does not yet know that which is called Love. Actually only the Soul knows, and is able to Love. Man has still not incarnated his Soul. Man still does not know what it is to Love. Love can only exist from heart to heart, from Soul to Soul – Whoever has not incarnated his Soul, does not know how to Love. Satan cannot Love and this is what the human being has incarnated. The Perfect Matrimony is the union of two beings, one who loves more, and the other who loves better. Love is the best religion that the human being can profess. Desire is a substance that separates into many substances. These substances of desire manage to deceive the mind and the heart. He who despairs because his wife has left him for another man, was not actually in Love. True Love demands nothing, asks nothing, desires nothing, thinks nothing, only wants one thing; the happiness of the beloved.
Osho’s commentaries on Tantric sex :
“Hence, Tantra was created. Tantra is a scientific approach. Tantra is alchemy; it can transform your centers, it can transform the other’s centers, it can create a rhythm and harmony between you and your beloved. That is the beauty of Tantra. It is like bringing electricity into your house. Then you can turn it on and off whenever you want. And you can have a thousand and one uses of it; it can cool your room, it can heat your room. Then it is a miracle. These seven centers in you are nothing but centers of body electricity. So, when I am talking about lightning, don’t think of it only as a symbol – I mean it literally.
“In your body, there exists a subtle current of electricity, very subtle. But the subtler it is, the deeper it goes. It is not very visible. Scientists say that all the electricity that is in your body, if put together, can be used to light a five-candle bulb. It is not much. Quantitatively it is not much, quantitatively the atom is not much, but qualitatively… If it explodes, it has tremendous energy in it.
“These seven centers, these seven chakras Yoga and Tantra have talked about down the ages, are nothing but five knots in your body electric current. They can be changed; they can be rearranged. They can be given a new shape, form. Two lovers can be transformed so deeply that all their seven centers can start meeting.
“Tantra is the science of transforming ordinary lovers into soul mates. And that is the grandeur of Tantra. It can transform the whole earth; it can transform each couple into soul mates.””Tantra is the natural way to God, the normal way to God. The object is to become so completely instinctual, so mindless, that we merge with ultimate nature – that the woman disappears and becomes a door for the ultimate, the man disappears and becomes a door for the ultimate.
“This is the tantric definition of our sexuality: the return to absolute innocence, absolute oneness. The greatest sexual thrill of all is not a search for thrills, but a silent waiting – utterly relaxed, utterly mindless. One is conscious, conscious only of being conscious. One is consciousness. One is contented but there is no content to it. And then there is great beauty, great benediction.
“The questioner asks: ‘What is tantric sex…a sex which is a meditation based on certain techniques?’
“If you are too technique oriented you will miss the mystery of Tantra. It is pseudo-Tantra that is based on techniques because if techniques are there, ego will be there, controlling. Then you will be doing it – and doing is the problem, doing brings the doer. Tantra has to be a non-doing; it cannot be technical. You can learn techniques – you can learn a certain breathing so that coitus can become longer. If you breathe very, very slowly, if you breathe without any hurry, then coitus will become longer, but you are controlling. It will not be wild and it will not be innocent, and it will not be meditation either. It will be mind – how can it be meditation? The mind will be controlling. You cannot even breathe fast, you have to keep your breathing slow – if the breathing is slow then ejaculation will take a longer time, because for ejaculation to happen the breathing has to be fast and chaotic. Now, this is technique but not Tantra.
“Tantra is not technique but prayer. Is not head oriented but a relaxation into the heart. Please remember it. Many books have been written on Tantra, they all talk about technique but the real Tantra has nothing to do with technique. The real Tantra cannot be written about, the real Tantra has to be imbibed. How to imbibe real Tantra? You will have to transform your whole approach.”Now, one thing to be remembered always: if you are not very alert you may go on believing that you are moving into tantra, and you may be simply rationalizing your sexuality – it may be nothing but sex, rationalized in the terminology of Tantra. If you move into sex with awareness, it can turn into tantra. If you move into tantra with unawareness, it can fall and become ordinary sex. It has happened in India – because only India has tried it.
“All Tantra schools in India, sooner or later, were reduced to sex orgies. It is very difficult to keep aware…it is almost impossible to keep aware. If from the very beginning the discipline has not gone very deep in you, there is every possibility that you will start deceiving yourself. Tantra schools arose in India with great energy, with great insight. And they had something – because that is the last center humanly available: the seventh is superhuman, the seventh is divine. The sixth is the spiritual center.
“From sex to tantra: a great revolution, a mutation, is possible in man. And in the East, people became aware that if you become meditative while making love, the quality of sex changes and something new enters into it – it becomes tantric, it becomes prayerful, it becomes meditative…it becomes samadhi.”