"I don't suggest doing it, but I think "flipping" is one of those things that people instinctively think should be bad for the gun, but which really are not."
I suspect that a single wrist flip, or even a few, would not cause damage that we could see or tabulate.
I also suspect that newer guns would be FAR more resistant to it than much older guns due to better steels and better heat treating.
But, I really suspect that it's the same as popping your clutch.
A few times isn't going to show much wear.
But it's going to be cumulative, just as a steady diet of .357 Magnum out of a Model 19 can show cumulative wear that one wouldn't normally expect with a stouter gun.
I also suspect that the bigger the frame, the more likely it is to happen, and happen faster.
Ultimately, though, it comes down to unnecessary abuse of an expensive precision object.
It's stupidity in action.