Overview mode allows you to see where your magnified area is in relation to the "real" screen. It is perfect for when you are asking yourself 'where am I?' on unfamiliar web pages or in a new application. The magnified area (displayed below) is quickly identifiable as an inverted rectangle on the screen (see bottom image).
Switch from the magnified screen.....
...to Overview mode, and quickly identify where you are on the 'real' screen.
This feature is present in SuperNova: Access Suite, Magnifier and Reader Magnifier.