One of the areas in Photoshop that troubled me the most was the usage of Luminosity masks. These masks belong in the advanced realm of Photoshop, and are usually not well described.
What is Luminosity masks
Luminosity masks are the cornerstone of tone-based image adjustments. These masks provide a convenient way to select specific tones in an image which can then be altered as you sees fit. They allow for an individual approach to interpreting light, allowing correcting tones that shifted during image manipulation.
To really “get” the power of these masks I can recommend Greg Benz blog, which really offers “Zero to Luminosity Masks”. Though a number of videos tutorials he gives you the understanding of how to create the masks manually, furthermore he offers few free Photoshop actions which will ease manual steps needed to create these masks