I read an interview many years ago about a sensei and concussions. "Gee, we went to the demo and woke up in the hospital. Ha! Ha!", said the student. Flat out amazes me to even think about it. Funny how things change. Concussions used to be ha, ha, in sports too, until the doctors figured out just how much damage they do.
But I do know that all the UchiDeshi of Osensei, who later went abroad to spread aikido in Europe and America did cause such injury frequently in demonstrations or in class, mostly on Nikkyo and Sankyo. They have all mellowed-down (with age probably). But one has to understand that they were just repeating what has been done to them during their own learning.
I might debate the words all and frequently but it probably did happen some. This is a classic example of abuse. Dad hits the kid who grows up to be just like Dad hitting his kids. You only hurt the one's you love right?