In Yoga, especially in Raja Yoga, one practices Pratyahara which means the gathering in, withdrawing and cooling down of the senses.
The idea of Yoga is to remain withdrawn from worldly attachments and center attention on spiritual goals, or uniting with the Atman, your true Self.
So Kundalini Shakti, which is a psychic energy based at the lower end of the spine below the coccyx center is likened to the coiled snake, the Kundalini power. This Shakti energy is dormant cold energy, just like the snake is a cold blooded creature, not truly activated much by us, whereas on the other hand, our passions in the mind are hot and our past karmas which stem from samskaras in the mind chit are also hot.
What Kriya does, is to re-activate Kundalini and with Will take her cold energy up the spine where she gathers our hot passionate energies of earth desires and past karmas, swirls them around for a few seconds and returns them to be cooled and made dispassionate in the cooling energies of Kundalini.
Therefore Kriya should be COOL UP the spine, and WARM DOWN the spine.
How this was devised by the yogis so that it could be understood, is that we as humans are likened to a thermometer, with our hair and brain being the reservoir of mercury. When our body is turned upside down out hair is the root of the tree with our limbs being the branches of the tree, we are then also as a thermometer, with the heat rising, but when we are right side up, it means the heat is going down the spine. It is the same for the cool of the thermometer. It is in reverse when we are right side up - the cool comes down.
That is why in Kriya the ancient sages devised this technique to burn out one year of past karma for every half minute of Kriya technique.
Running the heat in reverse can be dangerous as can sexual Tantra. When men practice running the heat up the spine in Tantra, which can be likened to an internal ejaculation and also most use it as a real physical internal ejaculation, they may risk testicular cancers as most of the sperm rots in the testes and later in life causes disease. There are millions of Yogis bamboozled in to doing tantra on this sexual level. It may not only be dangerous on a physical level, but also at a psychic level.
The Kriya I teach is passed down from its originator Babaji, on to Lahiri Mayasha, then to Swami Yukteswar, then to Yogananda, then to Dr. Lewis, then to me and of course many others.