The kiai (literally: "spirit yell") is very important to karate. It involves tensing the diaphragm and expelling air through the mouth, making a short, sharp
sound. The word "kiai" is not yelled; the appropriate sound should be like: "eye", "high" or "eigh". The exact sound will vary from person to person.
The kiai occurs during basic techniques, kata and sparring. It has three purposes:
(1) to demonstrate fighting spirit;
(2) to intimidate an opponent; and,
(3) to tense the chest and stomach muscles.
Failure to make a strong kiai, at the appropriate moment, is regarded as an error.
In kata, there are generally two kiai. The placement is according to conventions set by the J.K.A. It is not really an error to add a strong kiai at some additional point, but this should be avoided in gradings and tournaments.
During basic techniques, students should always kiai on the fifth (go) and tenth (ju) techniques in each set. There should also be a kiai on the last technique of any set, when turning and when assuming any stance in preparation for a series of techniques.
During basic sparring, the kiai occurs on the last attack and on the counter-attack. In advanced sparring, the kiai occurs whenever the attacker uses a decisive technique. No attack is regarded as strong, or decisive, without the kiai.
A strong kiai can intimidate an opponent and create an opening to attack. It can also be used to cause an opponent to flinch or step back, when used prior to the actual attack.
It is said that a strong kiai can even be used to stun or intimidate an opponent, preventing an attack. At one time, there was even a martial art known as kiai-jutsu, which focused primarily on the use of the kiai (successful application would require a truly exceptional fighting spirit).
Finally, the kiai helps increase the power of an attack by tensing the appropriate muscles. And, it helps to absorb an attack, by expelling air; this makes the chest and stomach firmer and less susceptible to having "the wind knocked out".