It's pseudo science. It's primarily theoretical. "Overdoing bonesmashing" is a bit of an oxymoron because for the theory to be applicable at all, you have to create micro fractures in your skull (hence the name), which requires a lot of force.
The "temporary" aspect is because obviously slamming shit into your face to break it slightly is going to cause inflammation, so all they're doing is inducing swelling on the skin. But nobody has ever actually done it properly, and nobody will.
It's mainly a meme thing, or the only people taking it seriously are young kids.