From a post I did on another site:
What I will describe here is one veiwpoint on the kiai, so many will probably disagree. Although this is a long post, I'll try to keep it as short as possible by not going into intricate detail.
There's a lot of myth circulating around a kiai. Part of this is from MA history where a physiological reaction is explained from a spiritual perspective. This is understandable as there is much about the internal workings of the body that were uknown at the time.
Before talking about what makes a correct kiai and why, let's look at the physiological effects of a correct kiai. The effect on the practitioner is two fold. With a correct kiai, there will be a slight flush in the practitioner, the pulse rate elevates slightly, and in some cases, the hair on the back of the neck or even the arms will stand up. This is very similar to the reaction caused by an adrenaline rush. Some studies were performed that showed certain noises can produce an adrenaline rush in the body, regardless of the source. They also found deep regulated breathing allows you to "control" the adrenaline rush.
Note the effects of breathing in the following quote:
"As described by Jim Viceroy, a Chicago-area exercise physiologist and sports psychology consultant, the body works like a piston-stretching and contracting, storing energy and releasing it-while executing a volleyball slam.
Says Viceroy: "Your body goes through a series of cocking all your joints.
Your hips cocks and your trunk cocks, your elbow and your wrist-like rubber
bands. You store elastic energy, you create the most tension possible in the
body, and you increase the range of motion, including the chest. By taking a
nice deep breath, you're presetting your muscles, stretching them. And so
when you exhale, if you do it correctly, you'll get this whole marshaling of
all those muscles at the same time, and this generates enormous force.
Therefore you have more velocity, more power.
The effect is partially psychological; much like the kiai, the shout of
the martial artist, it breaks down inhibitions and intimidates the opponent.
Of course, the force generated by the kiai would propel a backhanded tennis
ball over the fence."
Comment- earlier in the article, they explain that a "nice deep breath" means breathing with the diaphragm.
Summary- The physiological/psychological effects of a kiai performed at the right time, help synchronize the movement with the kiai, firing the "cocked" muscles and creating a strong mental intent that breaks down inhibitions that would keep you from releasing your full strength.
The rapid release of breath in the kiai, coupled with the increaed adrenaline results in a noticeable jump in power (increased strength from increased adrenaline is a well documented phenomena). In adddition, the forced deep inhalation also helps the practitioner utilize the adrenaline rush in a "fight" response rather than a flight response. One of the things kata practice does is aid in teaching you to control your breathing and link it to your movements.
He did mention the effect on the opponent. Let's take a quick look at that. Short, loud noises create the fight or flight reaction. This happens in species other than man. Example of such noises would be the initial clap of thunder, a pot dropped on the floor behind you, someone sneaking up behind you and yelling 'boo' loudly.
Many times this will cause a
momentary hesitation before the flight reaction takes over. The same thing happens with a proper kiai. It will induce a temporary "flight response" in your opponent, which can cause a pause in their movement and thought processes. This gives you a great advantage. The only thing is, the more often you kiai against an opponent, the less effect it has.
I use this as a training tool with my students. I have literally stopped them in their tracks during ippon on either their attack or counter. I work with them and tell them to learn to continue in spite of the kiai. They get better over time, although a kiai at an unexpected moment can still freeze them. You can see it rob power from a more advanced student's technique when delivered unexpectedly. (Stand behind them and kiai).
The fact that a strong kiai breaks down inhibitions can be seen when teaching people to kiai. Hence- the teaching that a strong kiai= a strong fighting spirit. A beginners initial kiais are weak and ineffective. Making them stand and practice their kiai can often induce giggles and smiles in beginners. Ask them why and they'll say "it just seems funny". When they can get past the social conditioning (don't yell or raise your voice) and really cut loose with a good kiai, this mentally sets them up for delivering full power techniques. I get a chuckle when I see a beginner produce their first good kiai and actually startle themselves
Now onto a proper kiai-
First- the air should be forced out using the
diaphragm
Second- it should be SHORT. No Bruce Lee drawn out waahhh's, eeee's and oooohhh's. (sorry BL fans, but that was for the movies).
Third- it should be LOUD
Fourth- the sound should be produced without any consonants. The reason for this is making a consonant sound like the 'k' in kiai restricts the breathing and slows down the expulsion of air
Fifth- do NOT exhale 100% of yoru air. Anyone that's been hit with no air in their lungs will tell you it's worse than being hit with your lungs full. Keep 10-15% of your air.
Think of your kiai as a "Clap of thunder" or the sound of a gunshot. It should crack the silence like a whip.