face slapping can be a bit of an emotional one for people,and reactions can work out very differently to how it's planned it's almost always startling and unexpected when it happens even when you do know its going to be part of play, its also a form of humiliation and not everyone can handle that psychologically or emotionally.

when we had not been together too long we discussed it and decided we would go for it but we didnt 'plan' a specific time or scenario.
one particular night we were getting toys etc prepared ready to play with later but id also been acting up a little and had been warned about my attitude a couple of times, i answered back thinking he was still being serious with me and got a gentle slap across the face for the first time ever..it was poor communication between us both in all honesty,IH had decided to start play and used that as a means to bring it about thinking id be aware of that we'd played like that many times.
but it kind of went wrong lol in my past r/ships id had partners who would beat me etc and i flipped thinking it was going to happen again, i jumped up threw the ashtray at him which split inside his ear open pretty badly saw him stand up and blood all over the place came towards me and i charged into the kitchen got the bread knife and cowered in the corner convinced he would knock the s**t out of me!
as it happened he was coming to reassure me that he wasnt and hadnt intended to hurt me even though he was faint and wobbling all over the place but it took a lot of convincing on his part he'd done it in play and not for real, things could have ended up very serious that night.

so it makes me very nervous and im never fully comfortable with it and it's something i rarely do.
it can turn me on to a degree possibly because nervousness turns me on anyway but its one area i have to be careful with and need to know in advance that its going to be a possibility for my partners benefit too!