This tool comes from the tried and trusted world of photography. It helps you build tonal variation by brightening up the areas you paint on.
The same settings are available as for the brush tool (see Brushes) except the following settings are also available:
Choose whether you want the tool to brighten up dark, medium or bright areas.
This parameter defines the strength of the effect. The more you paint over the same pixels, the brighter they become (provided you keep releasing the mouse button between brushstrokes).