I do this to my gf sometimes, usually when I am really horny and feeling particularly dominant in bed. I usually wait till she takes me down all the way and when she pulls back for some air I`ll grab a handful of her hair and cockslap her with my dick on one of her cheeks, maybe rub some of the spit on my cock all over her mouth and chin while I`m at it, she enjoys it very much and likes for me to call her a dirty little slut or putana too hehe. Sometimes she will slap herself with my dick as well, always a pleasant surprise and quite a turn on when she does it.