I was stunned because I didn't even think I slapped them that hard :P. Like Toral, I was concerned with getting it hard enough but also with not actually causing them serious bodily harm. I remember bracing myself before my sibling hit me, too.
I slapped mine kinda hard :look :P. Probably helped that they slapped me first - at least I think so? I honestly don't remember, it's been....a while :blink. Probably still footage/film out there somewhere, if someone has it I would like to have it!
I don't think we had to petition anyone, but...