Kiai

good Kiai      bad Kiai One of things that draws passersby into a dojang is the yelling they hear coming from inside the dojang. What the people hear is the kiai being sounded by the Taekwondo students while they are training (a sound similar to that heard when opening a TKDTutor page). Just like the uniform, colored belt, and bow, the kiai is an integral part of Taekwondo training. When people hear a kiai, they immediately associate it with the martial arts.
Samurai warriors were renowned for their powerful kiai in battle�a startling war cry that was said to paralyze opponents with fear. A warrior who could summon a powerful kiai would rarely be viewed as weak or tired by his opponents. Unfortunately, most people think of the whining howl used by Bruce Lee or other movie martial arts is the sound that all martial artists make. Actually, the grunt or puff sound used by boxers is closer to a kiai than most movie martial arts yells. 
As a student of Taekwondo, you learn how to kiai properly so it reinforces your technique. You also learn to not flinch and freeze at sudden loud noises so you will be able to more quickly assess the source of the noise, whether it poses any threat, and what, if any, response is required.
Meaning of Kiai
The word kiai is made up of the ideogram "ki," which means energy or spirit, and the ideogram "ai," which means matching or unite. Many Far Eastern people believe a force flows through all things. This force is called ki in Japanese and chi (or qi) in Chinese. Therefore, kiai means "working with ki" or "harmonizing ki" or "uniting the spirit."  E.J. Harrison, in his book The Fighting Spirit of Japan, describes it as the "art of perfectly concentrating all one's energy, physical and mental, upon a given object with unremitting determination so that one achieves one's goal."
How to Sound
The sounds Taekwondo students make when sounding a kiai are "E-eye" or variations of kiai, such as "ya," "oh," "or-ya," "utzz," "kiyup," or "e-yup". The exact sound of the yell will vary from person to person. You should experiment to find the best sound for you. However, do not try to sound like Bruce Lee�keep it simple and traditional sounding.  Staying with the standard kiai sounds will keep you from saying something offensive in a foreign language. One famous Karate expert was well known for using the word "kusoh" for his kiai�it means feces in Japanese. 
When to Sound
The kiai is sounded:

• At the moment of impact of a technique, whether it be a block or an attack
• When absorbing blows to the abdomen
• At certain points while performing patterns; failure to sound a strong kiai at the appropriate place is regarded as an error.
• While free-sparring to signify a decisive technique. An attack without an accompanying kiai is not is regarded as a strong, decisive technique
• Anytime you want to accentuate an action you are performing

Just like what the military tells you about when to salute: "When in doubt, do it." Use a kiai every time you execute a technique. Depending on the circumstances, it may be a low volume kiai similar to a whisper, or it may be a blood-curdling kiai that even causes the spectators to tremble.
Purposes
The kiai serves several purposes, it:

• Shows your fighting spirit. When performed under stress is like crying out "I will not give up! I will prevail!"
• Focuses total concentration into the attack.
• Focuses timing, breathing, movement, and power.
• Increases the power of an attack by tensing the appropriate muscles. 
• Ensures the breath is not held during exertion. Holding the breath during exertion increases blood pressure, which may be harmful. Ever seen the bulging veins on a weightlifters temples?
• Forces oxygen into bloodstream because muscles need maximum oxygen to perform forcefully (first syllable "ki" forces oxygen into bloodstream).
• Tenses the body at the moment of impact of an attack (second syllable "ai" tenses the body). 
• Tenses the body at the moment it receives a blow to direct the force of the blow throughout the body. When you are attacking, you are generally moving forward an vulnerable to a counterattack. If you are hit by a counterattack you cannot absorb the blow as well as you might if you were retreating from the blow or even standing still.
• Helps absorb an attack. By expelling air, the chest and stomach become firmer and less susceptible to having "the wind knocked out." Also, letting a blow slowly force the remaining air from the lungs gradually absorbs the power of the blow. If kicked in the chest while holding the breath, the rise in pressure in the chest cavity may cause the heart to go into fibrillation (heart muscle starts trembling instead of rhythmically pumping), which is potentially fatal.
• May surprise an opponent and break his or her concentration.
• May intimidate or "psyche out" an opponent.
• May actually stun an opponent, preventing an attack. Once there was a martial art known as "Kiai-jutsu," which focused on the use of the kiai as a weapon.
• May be used to create an opening. When used prior to the actual attack, it may cause an opponent to flinch or step back.
• Help release maximum energy. For example, when lifting weights, at some point your body says this weight is too heavy to lift. However, there are two common methods, and the use of the kiai, to circumvent your body's good sense:
  • The most obvious method is panic, or being "hyped up." Everyone has heard the urban myth of a mother who sees her son working under his car just as the the jack holding up the car breaks. She rushes out and lifts the car off of him. This may or may not be true, but in law enforcement we see plenty of situations where a panicked small woman is able to resists the efforts of many large officers to subdue her.
  • The other method is to use some type of drug. The most notorious one is PCP. It was originally used as an anesthetic for animals but was later rejected. Its effects on people include psychosis and an inability to feel pain.
  • The method Taekwondo students use is the kiai. It momentarily blots out fear and indecision and short-circuits the body's safety mechanisms for a fraction of a second. This usually is not harmful because full strength is only exerted for a split second and it is not being exerted against a significant opposing force.
• Makes sparring judges take notice.
• Boosts the overall spirit of a class of students.
• May impress and deter other potential aggressors.
• Alerts others to your predicament.

Breath Control
Proper breath control during execution of a technique requires:

• Low volume, high pressure exhale during execution
• High volume, low pressure exhale with a momentary cutoff of the breath at the moment of impact 
• Normal exhale after impact

The kiai accomplishes this type of breath control. 
Kiai Composition
The kiai is composed of a high frequency first syllable (ki) and a low frequency second syllable (yup). 
Ki
You sound "ki" at the start of a technique. When sounding "ki," you forcibly expel air through constricted vocal cords using the diaphragm, which restricts airflow, increases air pressure within the lungs, and forces oxygen into the blood stream. This relaxes the body so all muscles may work together for maximum speed and power.
Ai
You sound "ai" at the moment of impact. During the execution of a technique, all your concentration and power is focused on the moment of impact and the sounding of the "ai." When sounding "ai," you release a burst of air from the lungs that relieves the increased pressure and tenses the entire body, especially the abdomen. With the abdomen tensed, the upper and lower body are solidly connected into one integral unit. This permits the body to transfer power from the legs to the point of contact and permits the body to transfer the reaction force quickly through the body to the ground and back to the point of impact before contact terminates. 
After the "ai" is sounded, you continue with a normal exhale. This disconnects the upper and lower body, relaxes the entire body, and allows it to quickly return to the on-guard position.
There are five types of kiai:

• the deep, low frequency kiai sounded at the instant of execution of a technique,
• the high pitched shout of victory,
• a mid-range tone used in the practice of resuscitation,
• a silent, internal tone used during meditation, and
• a combat kiai.

Combat Kiai
In Taekwondo class or in no-contact or semi-contact free-sparring, we kiai with the mouth open. But in full-contact free-sparring or an actual fight, opening the mouth makes you vulnerable to losing teeth or a broken jaw. So, in a combat situation, it is best to use a semi-silent version of the kiai and keep the mouth tightly shut�make a sound similar to the puff sound boxers use.
Example of Kiai's Effect
To see the way the kiai locks the upper and lower body into an integral unit, try to perform a full-extension push-up. To perform the push-up:

• Lie flat on your stomach on the floor.
• Fully extend the arms in front of the head with the palms flat on the floor
• Use the fingers to raise the hands up so they rest on just the extended fingertips
• Fully extend the legs with the feet about a shoulder's width apart
• Dig the toes into the floor so only the balls of the feet are supporting the legs
• Now for the tough part, do a push-up on the fingertips and toes

Most people are unable to do a full-extension push-up the first few times they try. They push up the upper body and the lower body, but the abdomen stays on or near the floor. To see how a kiai locks the upper and lower body into one cohesive unit, properly sound a forceful kiai and tense the abdomen at the moment you attempt the push-up. Most people are now able to perform the push-up.
 source:komodia.com