When doing this free-hand, you can adjust the speed of the movement during the shot. You can have a small pause in the movement at the beginning, during, or end of the shot. Whatever you have in front of the camera during the pause, will get a bit longer exposure and look more distinct in the image. The result of these short pauses, will look like double exposure. For this scene, a long steady camera movement looks like another photo: Trees in ICM on Myrstigen hiking trail, Brastad 1. In that shot I started a little lower and got some of the walkway too (the black at the bottom). To get the branches a bit visible, I went for a slower movement and started with a small pause, before moving the camera downwards. That did the "double exposure effect" you see here.