WHY WE KIAI!
Kiai (pronounced key-eye) is a Japanese term used in martial arts. The term kiai commonly refers to a short yell before or during a fight or technique. ‘Kiaijutsu’ is the Japanese art of using a kiai when using Martial arts. The proper use of kiai-jutsu involves concentrating on the use of the body’s energy more than it does just shouting. A sound is just an audible indication of good kiai (aligned body structure, focused intent, and good breathing). A kiai can be a silent, co-ordination of breath with movement. A kiai is done by quickly exhaling and at the same time making a one syllable sound. “I”, “HI” and “EE” would be good sounds to use for a kiai. You want your kiai to be short, but as loud as possible. However, a kiai should not be confused with a yell. A well-executed kiai will not cause the throat to feel to raw, as yelling would. The noise from a kiai is said to arise from the
abdominal muscles and diaphragm and should not be sounded merely from the throat. A relaxed and powerful exhalation can add power to movement. As well as the above, the kiai can be used to:

• Startle and demoralize your enemy (Battle Cry). It should startle your opponent. Even a momentary pause or blink by your opponent can be the opening that you can take advantage of. Use a kiai to help create that opening.
  
  
• Protect the upper body from a strike by providing an escape route for exhalation of air. It empties air from the lungs. This is good, because if you are struck after executing a kiai, the chances of having the “wind knocked out of you” are greatly reduced. By getting as much air out of your lungs as possible, it makes you a somewhat harder, denser target. Therefore, if you are struck after a kiai, it should not hurt quite as much.
  
  
• Protect the lower body by rapidly contracting the transverse abdominals and other core muscles, shielding the internal organs. Should provide solid abdominal support for striking techniques. Knowing when to use a kiai and how to properly execute and apply it are just as important as knowing the proper way to punch, kick or block - but when you add a strong kiai to these moves, then you will have a devastating technique!
  
  Article taken from the GKR website.