Cut out stiff card stock to the shape of the mountains and hold it while slightly moving up towards the sky and back towards land during exposure. Ideally one wants to keep the card below the skyline. Generally it is an unwanted side effect of burning in the sky and not wanted. I used to use that method but then took classes to learn about masking and pinning the masks to the negative to burn in the sky without the white line artifact.
Exactly! I'm a lazy burner and whenever I encounter a situation like this, I err to the side of creating a darker edge on the mountains instead of a lighter edge on the sky. The former is generally somewhat less obnoxious.
Sometimes you can get away with simply bending a piece of card stock and holding it in such a way that it follows the curve of the scene. It's surprising how often that makes a match that's good enough to not show up - at least as long as the burning is somewhat subtle.