The upwardly channelized semen is known as Urdhva-retas in Hinduism. Urdhava means upward, retas is semen. The word urdhvaretas has become a synonym for a brahmacharin (celibate), but it is really more meaningful. An adept, by the practice of Yoga, especially brahmacharya (celibacy), never allows the semen to flow downward, but turns it in the upward direction. By the practice of mudras like khecari, vajroli and yoni mudra, one may become an urdhvaretas. Shiva Bhagavan, the god of yogins (Yogeshwara), is an ideal urdhvaretas. Other examples are sages such as Kanva and hatha yogis such as Gorakhnath. By becoming urdhvaretas, one raises the prana, increases virya (vitality) and extends the life span.
It is said in Dhyana Bindu Upanishad (79-80) that khechari mudra stops the downward flow of the elixir oozing out from the moon situated in the topmost portion of the skull (talu-mula). It involves turning the tip of the tongue backward and inserting it in the passage above the soft palate (kapalakuhara), at the same time fixing the gaze between the eyebrows. It is said that by practicing khechari mudra like this, the adept can preserve his semen without allowing for ejaculation even if one is embraced by a sexually excited women (kamini). It is mentioned also that, so long as the semen is preserved one need not fear death. The text goes on further to explain (verses 85 and 86) that if, by any chance, ejaculation takes place, the adept can turn the semen back upward and preserve it by the practice of yoni mudra. These two verses are reproduced in Hatha Yoga Pradipika (III.42-43)
Source – Encyclopedia of Hinduism Volume XI page 65 – IHRF - Rupa