The background/track here is completely white because you are only seeing the finish line. It’s the same effect, but the finish line is solid white. So the background is solid white.

And the shadows, you gotta remember you are only seeing the slices of the shadows as they were at the finish line itself at different points of time so to might get part of an arm and part of a leg at different angles that are then stitched together.

Edit: this might be better… the shadows in front aren’t being projected ahead. They are just happening at the line earlier than the shadows behind the runners. The only physical dimension here is up and down, right and left represent time, not forward or backwards.