It is bound to the P key. Finally, it will allow you set the glue and transparency for the paint. Please see the layers section of the page for more information about glue and transparency. Shift -dragging constrains penciling to vertical and horizontal only. Command temporarily activates the path arrow.
Option temporarily activates the eyedropper.
Ergo, it is the tool of choice for fixing up small edge details, minor scratches and blips on photos.