For the benefit of the uninitiated, a kiai (key'-eye) is the traditional "shout" one hears in the martial arts - but it is much more. In fact, there is no adequate translation for kiai without a brief discussion of its main component, ki. Ki, or its Chinese equivalent, chi, is correlated to breathing. Spirit has an etymology that has roots with breath, as in respiration. Ki, spirit, and breathing are deeply connected. So coordinating one's breathing with martial arts techniques is imperative to mastering them. The kiai works its magic during exhalation, and just prior to a strike; breathing out allows us to generate maximum power. An added benefit of kiai has the potential to startle the opponent, while invoking our own passion and kokoro (fighting spirit).
An appropriate, well timed kiai should be used sparingly. It's important for a kiai to be as convincing as possible. In most traditional karate katas, two kiais make separate appearances, usually during a critical moment in the fight sequence. Since all katas start with a defensive move, the kiai represents an offensive conclusion delivered at the end of a series of techniques. There has been some misunderstanding of the real meaning of kiai; some regard it as unnecessary, or even phony. It's really up to the practitioner to give it significance, otherwise it just becomes a form of glorified yelling. For the dedicated budoka, the kiai symbolizes the manifestation of breath and technique, unified in an exultation of spirit.