As the Kundalini energy ascends along the Sushumna, passing centre by centre through the six centres, the yogi’s internal vision expands to gain access to realms and dimensions that are beyond the reach of the ordinary senses.
Finally, when the Kundalini merges with the Paramatman, which is the blissful being at the crown centre, the yogi is absorbed in an ecstasy that far exceeds the ecstasy any ordinary human being has ever experienced before.
This is the link between tantra, basically the subject dealing with the arousal of the Kundalini Shakti, and sex. The theory of tantra is that, given food and shelter, the sexual energy is the most enjoyable experience for all human beings. It is the same energy–the sexual energy–that is sublimated to travel in the upward direction, instead of the downward direction.
Which is why, so much stress is laid on gathering your sexual energies and keeping them together at the lowest centre where the Kundalini sleeps, which is known as the Mooladhara–literally translated, it is the root foundation.
However, this ecstasy that one experiences when the sexual energies are gathered together and made to awaken the Kundalini in order to reach the Sahasrara Chakra at the crown of the head, is a multiple orgasm which no human being has experienced anywhere down below in any one of his lower centres.
Chakra means a wheel–that which moves–which rotates. Therefore, it also happens to be the meeting point between the pranic energies of the human body and the energies of the psychic world which flow in from outside.