Select boundry under mouse.

If i select some polygon boundary, where ever my mouse is, it's going to select 'inside or 'outside' the boundary, depending on where the mouse is'.

Here's my selection

If my mouse is in the center of the selection it's going to 'fill' that inner area.

If my mouse is on the outside of the selection, it's going to 'fill' that outer area.

This is SUCH a handy, essential selection tool, i use this constantly when working in Modo.