[…] breathing helps to transfer energy through the body.
From a Zen point of vue, breathing is the link between mind and body, spirit and posture, mind and waza (technique). In Karate, breathing becames energy. Air contains the energy of life.
Energy transfer, tension-relaxation interval, and contrast
Every movement we make is achieved by muscle contraction. (…) if a muscle is already tensed, the release of energy is inhibited and the movement is (…) slower because as to relax then contract. But if the muscle is relaxed, energy is released much more quickly resulting in faster and more efficient movement.
Because we use all parts of the body simultaneously, not serially, energy is tranferred through the body from the vital center or hara. Therefore, to achieve maximum efficiency of energy transfer, the body should move as one piece, one unit.
While the body moves in one unit overall to produce maximum momentum in executing Karate techniques, forces are applied sequentially instead of simultaneously. So, to achieve wholeness of movement, the vital center must be developed. Essential for developing the vital center is to have a harmonious interaction of tension and relaxation.”
(Ali Darwish, in Contrast and Energy Transfer in Karate)