That's the one thing about the guys they don't mess about with trying to make things look real, they just do it, which is why Hosei-san looks more miserable every year.
Yeah, most actors would move their face to the side as the other actor comes close to simulate the effect, but the Gaki guys go further. I mean if they're getting their balls whacked by the Chinko Machines that they invented, a slap is well within their means.
I dunno how serious he was being, but on a talk show Housei seemed to say he went to the dentists once and on an Xray of his head they could see some damage, the talk show host asked "Maybe that head damage was always there," to which he was like "No it wasn't, what are you trying to say about me!?"
He said he's even started dreaming of how to approach Downtown about getting out of the yearly slap, similar to how he got out of Vs Moriman because of the damage to his head he got from the burdock roots. If he has to then I hope Chono terrorizes him in other ways still.