While other images used slow shutter speed to add things that were not there the whole time, this image uses slow shutter speed to partially remove something that was there. By only staying in the frame for part of the 4 second exposure, the subject becomes translucent as the background was exposed longer. While the composition is not necessarily strong it works well for this subject because it fits into the idea of an unposed "ghost" sighting.
You can read the story behind the image on the original submission page.

