This will tick off people that believe in it, but NO. it is not a supernatural force. I've spent the last few years researching this and have found many things that show its a result of natural developments in the body. For example, the breaking of a single board or brick in a stack can be duplicated with a machine- no spiritual force there. It's a matter of force, acceleration and how far the techniques is stopped in the top piece. I can literally spend almost and hour and a half lecturing on this topic, so I can't write everything here. As an example, I've taught aikido's "unbendable arm" to non martial artists in less than 2 minutes without teaching anything about ki. It's a matter of what's called relaxed tension and correct body alignment. Also you get better at it over time because the individual tricep muscle fibers become better at firing all at one time the more you practice this. The strength increase from this natural development alone is amazing. This is why a 70 year old man that's been doing this for 50 years can resist a 30 yr old muscle bound practitioner. Also, when you look at the videos the uke are all tensed up. This robs tremendous amounts of power from their efforts due to several physiological factors. I've ticked off a few "unbendable arm" practitioners by using the same principles, mentioned above, they use to hold their arm straight when I bend their arm. They first assume I have "strong ki". When I explain to them what I did, they get upset because the explanation is contradictory to their "belief system". Here's a post I made on another forum about kiais, stopping people with "ki" in a kiai and "killer kiai" There is no DOCUMENTED case of someone being killed by a kiai. By documented, I mean an incident where independent witnesses could verify the story. Independent means students of the person making the claim are NOT the source. What I will describe here is one viewpoint 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 unknown at the time. Before talking about what makes a correct kiai and why, let's look at the physiological effects of a correct kiai on the karateka. 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 increased adrenaline results in a noticeable jump in power (increased strength from increased adrenaline is a well documented phenomena). In addition, 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 your 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. Can it kill? I guess if the opponent had a weak heart and was untrained it would be the same as some people that have literally been "scared to death". Can a kiai stop an animal? Yes- under the right conditions. Often the initial fright reaction makes an animal freeze (tiger's roar as an example).[/i] Hope this helps.