Spanking is sometimes a great problem solver (only of she is into bdsm) it might instantly cheer her up then and behave.. (Joke)
But seriously I had a girl once who was a 'bit' of a masochist. She would like noting more than a good slap or her face with the palm of my hand (so not to leave bruises, she actually tought me how and where to hit. Although The spanking I found quite enjoiable) this was not really my cup of tea especially since she wanted me to slap her so hard she would pass out..never did that btw..even if she wanted me, that was something I wouldn't want to do, could't see the play part in knocking your girlfriend out .most of the play, spank on the ass or a slap i understand, but she eventually started really annoying me hoping I would get mure punch so to say, so that was that, stopped seeing her, but we also had lots of fun and learned a thing or two from her, and respect that community in general, but was never into real pain or humilliation stuff, hey but at least they /we experiment, for some it even may ha a therapuitic effect. And everyone was very acceping of each other and next to various games/play, seemed more in tue with feelings of others an very supportive of each other...But this is probably not the tread to go in further detail
I do hope se is doing well.and hope her head is ok tho and she never found a guy who would punch her lights out a few times a week...she liked the feeling.
So long story short in this case hittin my woman might have been not socially accepted but if we both wanted which I did not at the time for BDSM reasons that would be great. It would have made us happy, can't immagine she filing charges for me routinely hitting her lights out tho. So and if we wouldn't have stuf in writing about the arrangement, all I could say when police was infomed of a homely dispute: is that she alwys is asking for it!
And besides some of my lady friends started training MMA probably not good either to hit one of them, and they even might beat you up first

(maybe do a self defece class

)as It would probably send you to the ER..maybe a broken jaw or other bodyparts where you might not tapout?
Btw if interested in MMA there is a new documentary out about the life of GSP UFC champ, great guy, great story!
Anyway not a veryious serious post but reminded me of some interesting and unaual situalions.