How It works
Photoshop actually creates an image mask for you without you having to do all the detail work. It uses the mask temporarily as you adjust it, and then applies the mask to the layer when you commit the changes. The Extract interface lets you control how Photoshop sees the image you are working on and provides its best, calculated guess as to what you want to mask out of the layer based on your hints and the image information. The better your hints, the better the results.