In general, all systems of highest yoga tantra's completion stage involve the preliminary process of controlling the vital energies flowing through the two side channels, rasana and lalana, and redirecting them into the central channel, avadhuti. This is indispensible.
In this tradition [the six yogas of Naropa] the expressions "the inner heat, the foundation stone," is well-known. This is because in the completion stage yogas one uses the inner heat technology from the very beginning in order to collect the subtle life-sustaining energies into the central channel and thereby arouse the innate great bliss. This is the actual basis upon which all practices rely and upon which all later completion stage yogas are founded. This inner heat doctrine establishes this basis.
The practice of the inner heat doctrine entails directing the life-sustaining energies into the central channel. Here the energies enter, abide, and are dissolved. When one trains well in this technique, the strength of the experience has the power to give control over the loss of the bodhimind substance [i.e. the sexual drops]. Then, based on this power, one can rely upon a karmamudra [sexual consort] as a conducive condition to arouse the four blisses. On this foundation, innate bliss is aroused. Arousing this innate bliss is the purpose of the practices of the inner heat yoga and karmamudra.
Quoted from A Book of Three Inspirations by Jey Lama Tsongkhapa (1357-1419), Translated by Glenn H. Mullin and published in The Practice of the Six Yogas of Naropa (1997) Snow Lion.