Pain is a permission slip. You hurt me, I have permission to hurt you. You attack me vigourously, I throw you vigourously.
Of all of the rules that I have followed, or cast aside, this one seems the most universal.
So, when said sensei is snapping on painful techniques, is the student mildly approaching, leaving themselves open to what the sensei choses to do? Or is the student attacking with real intent...trying to cut down the sensei?