well the kiai lasts as long as the movement you are acxecuting, a strike in kendo generaly lasts from kamai to kamai (kamai strike follow through turnaround kamai). Reason that you don't do a short kiai is that your body will almost automaticly stop/slow down if you inhale during a the follow through because it brakes the move whereas if you keep exhaling you will get further faster.



EH that's what I make of it sofar