OK, we've all seen those lame Kung Fu flix where the people go a bit mental on the Kiai...but is there really something to the whole "blood curdling scream" thing?
I would say YES. Done properly, kiai accomplishes a lot.
However, the key (as with everything) is doing it properly.
Kiai originates in the belly, not in the throat. Anyone who has studied singing will tell you that the powerful voice professional singers have cannot come from just using the throat. Some heavy metal singers without formal training try that and their voices are ruined in just a few years (the excessive lifestyle probably does not help either).
The kiai sound is made by a sharp exhalation of breath from the diaphragm combined with focus of intent behind it. It takes practice, real practice to be able to do this properly, since most of us are socialized not to use our voices in such a way (LOUDLY, that is).
When it works it...
1) Disconcerts Uke
This loud sound causes an involuntary reaction of Uke to blink or flinch. This creates an opening into Uke for a technique to manifest (if you do not consider kiai a technique on its own already).
2) Focuses your Intent
Projecting your voice has the added benefit of helping you channel your own intention. We often see a similar usage by weightlifters and other athletes when they exert effort. Aikido is all about channeling intention.
3) Contracts the Muscles
This is part of kime, focus, and adds a certain extra "snap" to techniques. Of course, by flexing, you can then relax, which is an important part as well. Kiai should occur just at that moment of tension, and then release lick a bullwhip cracking.
4) Affirms your Sense of Being
Kiai is very primal. It affirms us as ALIVE, and helps to shake us awake from the dream that is our daily routine. We know that our study of aikido is designed to help us become "more alive", and kiai is one very great example of this. It is, quite literally, a projection of our sense of self.
Half-hearted kiai indicates half-hearted aikido, which indicates a half-hearted sense of being. We should strive to be 100% in the moment during our training (which helps us be 100% in the moment outside the dojo as well).
Our laboratory, the dojo, is a great place to develop this technique, where kiai helps to create a cadence in the warmup, or to add that extra focus during practice. I think of Kiai as one of the atemi techniques in my aikido arsenal, and a very useful one in combination with other techniques I do.
For students who don't like it or refuse to do it, I would encourage you to reconsider.
This SHOUT OUT could help add another dimension to your aikido.
(hear) you on the mats,