Some people in the West first heard about Kan and Li as Taoist meditation from master Mantak Chia. Kan (坎: 010) and Li (離: 101) are the bottom and top trigrams when the eight trigrams are arranged in a circle. There are many metaphors to represent the concept of Taoist mediation and Kan & Li being one. The use of metaphors can create a mythical feeling more conducive to getting results from deep meditation. The issue for the modern man (meaning one who, fortunately or unfortunately, is rational and scientific in orientation) is how to be benefited from a deep meditation system despite his rationality. To achieve this objective, a modern man needs to understand the logic behind such metaphors. So, why Kan and Li?
In I-Ching, Kan signifies female, worldly, and water, with a bit of male (signified by "1") inside; Li signifies male, spiritual,and fire, with a bit of female (signified by "0") inside. In Taoist meditation, step one is for Kan to send "1" to fill/replace Li's "0", step two is for Li to send "0" to fill/replace Kan's "1", and step three is for Li and Kan to merge together to ultimately form Tao.
The above shows the gist of the concept. The question is: how does it relate to Taoist meditation practice? In Taoist deep meditation, it is firstly the mind controls the restless body, then the controlled body controls the restless mind, and finally mind-body combines as one.
If you think that I'm over simplifying things and doesn't understanding the hidden secret of the practice, I'm not. Some prominent Taoists of the later Dynasties (like Ming, Ching) had already started to demystify some ancient writings (in particular the mythical language of Can Tong Qi), and used different simplified concepts to teach their practice. This demystifying approach is more important nowadays, lest sound practices will become too esoteric for any scientific person to pursue. In future posts, I shall try to explain some of these more easily understandable systems for the benefits of fellow Taoist meditation practitioners.
PS: My interpretation of Kan & Li is different from what Mantak Chia was saying in his books. Needless to say, I write what I understand and believe to be of more explanatory value, and more conducive to a better practice.