As soon as you create a layer mask, it acts on the image. This option allows you to abort this action. The thumbnail border of the layer mask in Layer Dialog turns to red.
You can access to this function by checking
-> -> ,or by holding Alt key down (Ctrl-Alt on some systems) and clicking once on the layer mask thumbnail in Layer Dialog.
You can undo this action by unchecking the option in the Layer/Masque menu or by (Ctrl)Alt-clicking again on the layer mask thumbnail.