The negatively polarized sexual energy.

"The Lord sent against the people the fiery snakes [הנחשים השרפים], and they bit the people, and many people of Israel died. The people came to Moses and said, "We have sinned, for we have spoken against the Lord and against you. Pray to the Lord that He remove the snakes from us." So Moses prayed on behalf of the people." - Numbers 21:6

"It is necessary to know that the Kundabuffer Organ is the negative development of the fire. This is the descending serpent, which precipitates itself from the coccyx downwards, towards the atomic infernos of the human being. The Kundabuffer Organ is the horrifying tail of Satan, which is shown in the “body of desires” of the Intellectual Animal, who in the present times is falsely called man." - Samael Aun Weor, The Elimination of Satan's Tail

"The diabolic type whose seduction is here, there and everywhere under the pretext of working in the Ninth Sphere, who abandons his wife because he thinks she will not be useful to him for the work in the Fiery Forge of Vulcan, instead of awakening Kundalini, will awaken the abominable Kundabuffer organ. A certain Initiate, whose name will not be mentioned in this Treatise, commits the error of attributing to the Kundalini all the sinister qualities of the Kundabuffer organ... When the Fire is cast downwards from the chakra of the coccyx, the tail of Satan appears; the abominable Kundabuffer organ. The hypnotic power of the organ of Witches’ Sabbath holds the human multitude asleep and depraved. Those who commit the crime of practicing Black Tantra (Sexual Magic with Seminal Ejaculation) clearly awaken and develop the organ of all fatalities. Those who betray their Guru or Master, even if practicing White Tantra (without seminal ejaculation), will obviously activate the organ of all evils. Such sinister power opens the seven doorways of the lower abdomen (the seven infernal chakras) and converts us into terribly perverse demons." - Samael Aun Weor, The Mystery of the Golden Blossom