I don't know for sure it was a concussion, but it was one of the hardest throws (as uke) I've ever experienced. After class, I questioned one of the senior students about a technique we were doing (katate mochi, sokumen iriminage). He asked me to grab his wrist and push. I did, and he said 'no, really push!' I did, and when I woke up, he was staring down at me asking if I was alright.
Lesson # 1, don't ask questions
Lesson # 2, don't grab his wrist
Lesson # 3, NEVER push harder than you can fall...
Jun, I think you know who was shite...
Ron