Layer masking
Most photographers will agree when I say that you can only take your editing so far using Elements' global tone-changing tools. In this book, we have seen how effective the global tools are, but sooner rather than later, you'll realize that you need to develop skills to edit small parts of your picture, rather than the entire file.
Layer masking is one of the keystone features of all photo editing applications for the simple reason that it allows you to make significant changes to very specific parts of any image. Once set up, it's easy to do and is infinitely editable. This is achieved by first making a global edit (using a tool such as Levels); then, with the mask active, painting black makes the layer transparent, thus revealing the original state of the image. Painting white makes the mask opaque, thus covering the original image with the newly edited version.
The magic of this technique becomes apparent if you make a mistake and paint over something...