As with the other point-adjustment functions, when this button is
pressed, if the selected object is a polygon, the bounding-box and its
attributes will be removed, and a series of coordinate points will
appear. Pressing Mouse-Button-1 in any of these points will delete
that point from the polygon.
Once again, there is no Undo button at present, so take
care. If you do make a mistake, you can use the Add Point button
to amend it.