The Position Pass can be used in 3D-compatible compositing applications that can in turn be used to create all sorts of effects with it.
In short, this pass contains 32-bit encoded positional information. The composition application knows the location of each pixel in 3D space and can - depending on the application itself - calculate effects such as new lighting, 3D masks or other three-dimensional effects.
OpenEXR is the recommended format.
Note that Photoshop CS5 often has difficulty handling negative values and also cannot correctly read OpenEXR layers.
Make sure of the following when using this post effect:
Edge antialiasing has intentionally been omitted from the Position Pass function because this would render it unusable.