An important art, which forms a part of many systems but is rarely perfected on its own, is that of Kiai Jitsu ('The Technique of the Shout'). Kiai is the name given in Japan to the yell that is often used in combat. 'Ki' is the internal energy, and 'ai' means union or integration, thus the Kiai is a total unification of energy, a full, instantaneous focus of all the faculties of one's being into one act. The Samurai say, 'One stroke, one life'.
Each act, to be a true act, should have one's entire being focussed in it. This is the spirit of the Kiai, which is the expression in sound of this unification. The shout is generated from the lower abdomen, the hara, which is the seat of the vital power. It emerges spontaneously and without obstruction from the depths of one's being and is directed by clear intention to merge with the target. Indeed in the Kiai, the shouter, shout and target become one; it is not a case of a man trying to shout very loudly, so as to influence someone a distance away. Rather, in that transcendent moment of total intensity, space itself collapses and only the Kiai exists.
The Kiai can be used for many purposes; the intention can use the focussed energy to heal or to hurt. In some Judo dojos (schools) the use of the Kiai to revive an unconscious person is taught along with more orthodox percussive massage (Kuatsu). One is reminded of Christ commanding Lazarus to arise from the dead; the magic and power of the word; the science of mantra, in the beginning was the 'Word' vibration as the essence of the creative process.
Some have attempted to analyse the action of the Kiai in terms of modern neurophysiology. One can trace neural links, which allow the heart and other organ functions to be affected by sound stimulation. I find this very interesting and have no doubt it is part, but only part, of the story. Every new scientific discovery offers new explanations for ancient techniques - and if nothing else is certain, it is clear that the science of twenty years hence will have new, different and probably (I hope) more profound explanations. According to Musashi (perhaps the greatest Samurai in history), the Kiai is used before a technique, to gather power and to awe the opponent.