When the melting of the bodhimind substances [sexual energy] is accomplished on the basis of having dissolved the energies into the central channel [of the spinal column], then the drops [of energy] can be retained at the tip of the jewel [sexual organ] until the innate bliss is aroused.
The drops should not be allowed to be ejaculated. This retention will only be possible if the energy that causes ejaculation has been absorbed [into the central channel].
In the beginning of the practice, in order to retain the drops in the jewel chakra and arouse the innate bliss, it is useful to visualize that the drops are directed into the central channel. Engage in the energy practices until this ability is achieved. Don't overly exert force.
Quoted from A Practice Manual on the Six Yogas of Naropa:Taking the Practice in Hand 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.