One important category of neidan practices are what are known as Kan & Li practices. Kan is the Chinese word for water; and Li is the Chinese word for fire – so Kan & Li practices are Water & Fire practices. In the Five Element System – as employed in Chinese Medicine as well as qigong practice – it is the relationship between Fire and Water, between the Heart and the Kidneys, that is considered of utmost importance.
As far as I know, Mantak Chia was the first to make Kan & Li practices openly available to western practitioners. These are advanced practices, and so typically an Inner Alchemy practitioner engages in them only after s/he has become adept at foundational practices such as the Inner Smile and Healing Sounds. It’s also important to have the guidance of a qualified teacher, in order for Kan & Li practices to be safe and effective. What follows is just the briefest introduction to these powerful practices.
Forming An Immortal Child (Fetus)
In Tao & The Tree Of Life Eric Yudelove – a student of Mantak Chia -- writes:
In the Kan and Li exercises, the student of Taoist Yoga actually forms an Immortal Child within himself. This Immortal Child is carefully grown and nurtured and then transferred first into the Soul Body and then after much work into the Spirit Body.
Lesser Kan & Li – Creating The Soul Body
The first step in Kan & Li practice involves learning to transform -- via a "steaming" process -- sexual energy (Jing) into life-force energy (Qi). This transformed life-force energy is then used to nourish a subtle energetic body called the Soul Body – an aspect of ourselves that is free from environmental & karmic conditioning.
Greater & Greatest Kan & Li – Creating The Spirit Body
The highest level of Kan & Li practices are used to catalyze the formation of an Immortal Fetus, and transfer it into a Spirit Body -- a very subtle body which can function independently of the physical body, and into which one can transfer ones consciousness at the moment of death. Highly accomplished practitioners are able also to transfer part or all of their physical form into this subtle body – making it more-or-less equivalent, or at least similar, to the Tibetan Rainbow Body practice.
Fire, Water, Stove & Cauldron
What is the source of the Fire and the Water – the ingredients of this alchemical process? In Lesser Kan & Li practice, Fire-energy and Water-energy are drawn from within the body of the practitioner, en route to creating the Soul Body. In Greater and Greatest Kan & Li, Fire-energy and Water-energy are gathered from outside the body of the practitioner, en route to creating the Spirit Body.
In all Kan & Li practices, the Fire (whose nature is to rise) is placed below the Water, creating what is known, in Inner Alchemical parlance, as the "Stove." At the same time, the Water (whose nature is to sink) is lifted into an imaginary Cauldron – a cooking-vessel which rests on the Fire. The Water in the Cauldron is "stirred" until, in combination with the heat from the Fire, it turns into steam.
The Lesser, Greater and Greatest Kan & Li practices differ in terms of the source of the Fire-energy and Water-energy; where the Cauldron is placed; and the source of the "seed" for the conception of the Immortal Fetus. Here's a summary:
Lesser Kan & Li: Fire is drawn from heart, and Water is drawn from genitals. The Cauldron is at the level of the navel. The Seed is from the liver.
Greater Kan & Li: Fire is drawn from the sun, and Water is drawn from the earth. The Cauldron is at the level of the solar plexus. The Seed is from the spleen.
Greatest Kan & Li: Fire is drawn from the North Star, and Water is drawn from moon. The Cauldron is at the level of the heart.
The steaming process that occurs as Fire-energy merges with Water-energy tends to produce feelings of great sweetness, joy and ecstasy within the bodymind of the practitioner. Because of this, this part of the practice is sometimes referred to as "self-intercourse."
Eric Yudelove describes this streaming process, as well as the seeding of the Immortal Fetus:
The steam generated from the boiling cauldron is drawn to the genital region and mixed with the sexual energy found there. The mixed steam and sexual energy is then raised up in the body. It is drawn to certain internal organs and the effect can only be described as having sex within the body. The process is known as Self-Intercourse and has long been one of the most closely guarded secrets of the Taoists. The organs provide a seed (the liver in Lesser Kan and Li and the spleen in Greater). In Greater Kan and Li if successfully performed, the seed is actually impregnated and a spiritual fetus is created. In Greatest Kan and Li the fetus matures and is transferred into the Spirit Body – but it is not yet mature enough to go out on its own. In the Sealing of the Five Senses, the cauldron is drawn to the brain. All the senses are sealed and complete consciousness is transferred to the immortal child.