From what I remember (although I forget where I read it, probably on this very forum) Falk was supposed to walk down and he fell for real. He thankfully wasn’t hurt so they kept it in. And....that really WAS the quickest way down the hill.
I've been having a look at it again... Well, if he doesn't really fall without wanting to, he's a better actor than I thought (and it's difficult to think higher of Mr Falk's abilities as an actor than I do)!
I thought the fall had been shot all in the same go, but although there's a cut in the middle, to me it looks like the same fall filmed using two different cameras (I'd say the final part from the first camera is the same as the first moments from the second camera).