I have been living with this revelation for some time now, and one of my observations relates to this comment by Philip:
Basically, kiai are placed at kime-waza - decisive techniques. KO, or more likely killing strikes (todome). For a student to perform a kata perfectly, with correct timing, intent, and visualization of their opponents, it is vital that they understand the applications, where the todome moves are, etc.
This is the legend they tell us when we are learning Heian Shodan, and it seems very convincing at that point. If you look at the other heian katas, however, you'll be hard-pressed to believe that the kiais are placed on powerful killing strikes. They just stuck the heian kiais on the move before the major turns, except in a couple of places where there wasn't a shred of a justification for it. The second kiai in Heian Nidan, for instance, would have been attached to a reinforced block. After looking around for another move to stick the kiai on, they gave up and stuck it on the last move of the kata, an upblock. If you don't know about the swords in that kata, a "kiling technique" is kind of hard to find.
Yes a few of the kiai moves are killing techniques, but many/most are not. They are just yells to make the kata more dramatic.
And then Peter:
When I read the first edition of Shotokan's Secret, I can remember one chapter explained that the kiai has been a "mark" for the comrades... so that the comrades who are responsible for the king's life, know, when the disorientation of the aggressors has reached it's peak, or when the fighter comes back to them...
So my question is:
Is this thesis completely thrown away now and the kiai is marked as a shotoism, or are there still some paths of information to follow?
I do not have experience with weapons fighting (sword, bo, ...) but I could imagine to "yell" when I have to use a heavy sword. Are there kiais in Iaido/kendo or similar martial arts?
Well, first, the theory of the kiai being a signal among the bodyguards fell flat when it turned out that they didn't use kiai in their katas. As we all realized, kiai isn't very stealthy, so that makes good sense.
Second, Shotokan kumite is patterened on kendo, and in that sport the contestants do indeed kiai as they attack. I imagine that is how kiai entered karate, and eventually crept into our katas.
Third, it is important to understand that kiai has various benefits, both spiritual and biomechanical, and we could talk a long time about what those benefits might me. However, the point being made here is that kiais were added to the Shotokan kata at arbitrary points to increase drama. Some of those arbitrary points were killing techniques, but a lot were not.
So don't lie awake at night trying to figure out how the jump in Heian Godan could be a killing technique. Neither the kiai nor the jump were part of the original kata. Both are shotoisms.