Using this mode, you can modify (or create) a selection using pixel-editing tools.
When entering this mode, your selection is temporarily presented as a pixel mask. Pixels can be added or erased using the standard painting and erase tools.
By default, while in this mode your page will show with a translucent, red overlay. After painting in white, the resulting untinted areas will not be included in the selection. This default view can be changed depending on your preferences.