The native sample is a simple wxWidgets application that displays the following form:
The Visible/Hidden radio buttons toggle the visibility of the overlay surface. When you select Visible, the overlay surface will be created and shown to the top-left of the screen, containing a snapshot of the sample window. The sample updates the overlay surface
updates twice per second. To illustrate this, there's a text box that you can edit, and watch your changes as they're reflected in the overlay.
Given that overlays aren't captured by PrntScn, I made the following mockup to give you an idea of what to expect:
