This plugin is for isolating specular light in a scene. It uses two photographs of the scene, one with specular light filtered by a polariser and the other allowing it through. The former is subtracted from the latter to leave mostly specular light.
The plugin does the subtraction using layers, which can be done onto a separate layer if needed. Both images need to be aligned to overlap accurately and their layers can be moved to do this if necessary. Some noise is likely and there's a controller to reduce this, it defaults to 100% and is best left at that. To keep colour distortion to a minimum the photographs should be exposed in the mid to high range, avoiding exposure near white. A brightness control is included to compensate for this.
Getting good results can be tricky, I've put a blog post together which outlines some gotchas and can be found here: