In kendo there is an emphasis on group unity in the warm-ups and initial drills. In my dojo (NW Kendo), our sensei is looking for us to have loud and powerful kiai in unison early on so that, by the time we are in jigeiko (individual bouts) our spirit is powerful and the kiai remain loud and sharp because that keeps one’s energy up so as to maintain performance and have a tactical edge over the opponent.
If that sounds esoteric or just not compelling enough, consider this lesson on group unity from Jocko Willink (@jockowillink). When his unit was deployed in Iraq and were going out on a patrol or assignment, they had a tradition of starting up their truck and jeeps at the same time. According to Willink, that sound created a powerful feeling and sent shivers down their spines; creating a sense of power over their enemy.
So start your engines!
Sumi Sensei instructing on kiai:
Mondo 