WASD to move background object. Try moving around with Mario (Arrow Keys). This directly connects to the scrolling and is meant to be appear as if it can go to background layers (i.e. the waterfalls). I tried something sort of similar to this in MMA2 in the pyramid level but that was mostly just resizing Mario's sprite and playing around with layer blocks. If you look closely at that you'll realize Mario is connected to the foreground scrolling rather then the background. Here the sprite appears as if it actually gets further away from the camera.
@Brad-Games this is modified from SMFS4 unfinished.