Selections and Cutouts - The Photoshop: Professional Retouching and Compositing Tips, Tricks, and Techniques - page 28

Figure 1.28
4. With the red overlay it's easy to see what is missing from the selection, such as the
underside of the arm and part of the pants. To include these areas in the selection,
paint over them with a black brush at around 75% hardness; to remove areas that
should not be part of the selection, paint with a white brush at 75% hardness.
5. Press Q or click the Quick Mask icon in the toolbar.