The tanjun is the center of the body (considered to be at the center of the torso behind the belt knot at the center of mass). It is the source of ki, which is mental, intrinsic, or breath energy, as opposed to muscle energy. All taekwondo power emanates from the tanjun and flows through the body as ki. Think of ki as water. The tanjun pumps ki into the body. If the ki is pumped into the body cavity, it will pool there and lose energy just as water in a tank. However, if the flow is directed to a specific area, similar to water flowing through a hose, the energy will be available at the end of the hose.
A properly executed technique will conduct ki directly to the point of impact. If the technique is not executed properly, a break may occur in the hose and ki will leak into an undesired area, such as a ki flowing into the shoulder instead of to the striking hand. When the flow of water divides, the force of the water is weakened in each branch. If ki is directed to more than one area of the body during the execution of a technique, the technique will not attain maximum power. Maximum power in taekwondo is attained by concentrating the flow of ki from the tanjun directly to the point of impact.
Ki flow must be timed so it reaches the point of impact at the exact moment of impact. If it reaches the point of impact too soon, it cannot flow into the opponent and will be reflected back into the body, causing a disruption of ki flow throughout the body. The principle is illustrated by watching a lumberjack chop wood. The axe is swung in a smooth arc with the body relaxed and the muscles acting to progress fully increase the velocity of the axe. At the movement of impact, the ki is released, the body tenses, and all the body's energy is directed to the edge of the blade. If the ki flow does not occur, the axe may bounce off the wood or not penetrate deeply.