The undercutter produces a rabbetlike recess on the back of the panel. This allows you to use W'-thick stock for panels without having to cut excessively deep with the panel-raising bit. An undercut panel ends up being flush with both the face and the back of a standard frame. An undercutter can be purchased as a separate bit. After raising the panels, use the undercutter to relieve that panel back as necessary to size the tongue.