Here comes a video that explains how the Layered PSD files works and a bit of what you can do with them.
To make full use of them you need Photoshop. I tested the files in GIMP but the usability is limited. GIMP doesn't support adjustment layers so the shading will look dark or washed out depending on how you brighten it.
I will look into trying to see if there is a way around that limitation in GIMP. It would be neat since not everyone can afford to use Photoshop. Will also test it in Photoshop Elements to see if the more affordable version can handle it.