This will help to restore highlight detail on a slightly over-exposed image. It works well after using a contrast enhancing effect. You will paint this in.
Paint over areas that need to be darker. Won't affect midtones and highlights. Great for eyeliner and those dark lines around the iris of the eye. What do you call those anyway?
Use to interactively enhance certain color ranges, adding a natural vibrance without looking over-saturated. For example you could make blue skies bluer.