Lama Ole’s answer:
A meditator’s goal is to have the same control of his body that a racecar driver has of his car. That is, we become capable of steering the energies in the way and direction we choose. However, only those who have already accomplished a lot can do this practice. It is necessary to train, to learn to experience things in a beyond-personal way, and to develop meditative power.
If we look at the succession of the practices, the tantric union teachings come after the practice of “inner heat,” where one can sit in the snow and melt it. To do this correctly, one must be able to pull the energies down, hold them, and turn them around. There is a yogi joke that if a man has truly accomplished this practice, he can pull water mixed with milk up through his penis and then release the water and milk again separately. I have no idea to what extent this is true, but in any case, the practice is very difficult indeed.
The reason we don’t talk a lot about sexual union practices isn’t prudishness. From the Buddhist viewpoint, the body is neither dirty nor sinful. It is a tool—a temple of energy where enlightenment can take place. But one explanation of why union practices are not mentioned often is that many people have difficulties concerning sexuality. If we talk about the energies of the body, sexual energies, and working with them, people often start to think in their old-fashioned Christian or Islamic concepts, regarding sexuality as something dirty and wrong. They build up dualities.
The union practices are generally kept secret for the people’s own good—because they simply cannot understand what a jewel they are holding. If we still hope and fear, hold on and push away, if we want to keep ourselves under control and develop disturbing feelings, then it’s better to keep the lid on it. Only when we can meet each other in a state of giving and taking and aim at developing each other, only then does the tantric union practice become useful. Not until one experiences everything as a dream and can see both practitioners as forms of energy and light does it make any sense to invoke the powerfields and exchange energies.
Generally speaking, it’s like this: use your body as a tool to give others joy and avoid causing them suffering. You should also not destroy the joy experienced when making love with any ideas about how we should do it. Enjoy, be good to your partner, and everything else will evolve gradually.
In Buddhism, the ultimate level of insight is called detong, which means the inseparability of joy and emptiness. That means that you remain in a state of bliss and ecstasy. You keep this state without interruption, and you are in full control of it. But it is not a personal state. You radiate into space, full of joy. As soon as something is touched by you, it starts to radiate too. In this state, the experience isn’t that one person is doing something for somebody else. Rather something happens in open space. Try to hold this level all the time, when making love too. Experience it as the highest initiation and not as a small thing you only want to keep for yourself. Then it will be an encounter full of highest truth and the most beautiful jewels. Stay aware of this and then radiate on everything.