Ch'i is an

intrinsic energy, or life force. While invisible and intangible, it permeates all living creatures, and is inseparable from life itself. Almost all Eastern martial arts include exercises to develop, cultivate, store and channel ch'i. An adept practitioner can channel his ch'i to vastly augment his/her strength, endurance, and destructive power.
Simplistically again, the smooth flow of ch'i is regulated by two cycles.
• The Sheng cycle produces and augments the different types of chi within the body. In esoteric terms, metal melts to produce liquid, or water; water in turn nourishes plants (wood); wood adds fuel to the fire; and from fire comes ashes, or earth.
• On the other hand, the Ko cycle causes the mutual retardation of the different types of energy. Metal cuts down wood. Water destroys fire. Wood overruns the earth. Fire melts metal, and earth soaks up water.