This method can really make a good selection. I often use it to select the background and then blur it making the subject seem even more sharp.
It is also useful for creating basic shapes. For instance, you can make a cresent shape by making a circle selection and then subtracting a smaller circle. Or make a half-circle by making a circle selection and subtracting half of it with the rectangle marquee tool. Or make those famous mouse ears by adding three circle shapes.
Photoshop Masks & Selections
Compatibility: Photoshop (any version), Photoshop Elements