(Sanskrit, from vajra: "thunderbolt, lightning, diamond, adamantine") 

A practice common to Hindu and Tibetan Yogas, albeit with an incredible amount of variation and opposing uses. As described by Swami Sivananda in his book Kundalini Yoga:  "This is an important Yogic Kriya in Hatha Yoga. You will have to work hard to get full success in this Kriya. There are very few people who are experts in this act... This Kriya is of immense use for keeping up perfect Brahmacharya (chastity)... Even a drop of semen cannot come out of the Yogi who practises this Mudra... The Yogi, who draws his semen up and preserves it, conquers death. Good smell emanates from his body... The object of Vajroli Mudra is to be perfectly established in Brahmacharya. When aspirants practise this Mudra, they unconsciously divert their mind to sexual centres and thereby they cannot get any success. When you see the description of this Mudra, you will clearly understand that strict Brahmacharya is absolutely necessary... This must be done under the direct guidance of a Guru."

Samael Aun Weor indicated that certain aspects of the traditional Vajroli are damaging to the organism. He taught a safe method in the transcribed book "The Tibetan Exercises of Rejuvenation."

"It causes us great horror to know that the tenebrous black magicians of the Drukpa clan (those who are dedicated to the fatal and horrible black tantrism) ejaculate the seminal liquor during their practices of black magic. These black magicians have a fatal technique that enables them to reabsorb the spilled semen. Such a technique is known as black vajroli. This practice is completely negative; therefore we do not even want to explain the procedure. We know that there exist many people with very weak mentalities, who could easily be led into practicing such horrible black tantrism. Karma would then fatally fall upon us. The spilled semen is mixed with feminine “verya” (feminine semen) and afterwards it is re-absorbed. Thus, it is horribly recharged with atoms of the secret enemy. These are satanic atoms that are collected from the atomic infernos of the human being. The inevitable result of this black tantrism is the descent of the serpent towards the atomic abyss of Nature. This is how the human personality ends. It is definitely disconnected from its Divine Spirit. The human being is then transformed into a demon. The Arcanum A.Z.F. (Urdhvarata) was practiced within the ashrams of ancient India. At that time, yogis were prepared for sexual magic with the white vajroli. Unfortunately, the brothers and sisters of the temple committed scandalous acts, which discredited sexual magic. The Gurujis then pulled the curtains of esoterism shut, and the Arcanum A.Z.F. was forbidden." - The Yellow Book