You will now use the background eraser to totally eliminate the sky.  If you set the sampling (in the toolbar above) to "Continuous" any foreground color in the crosshair will be deleted because you have designated it as the target color.  The "Once" sampling option allows you to choose a color to erase when you first begin dragging instead of continuously choosing colors in the path.  This allows you to be more careful and detailed.  It is particularly useful when you have a background that is mostly one color.