If you choose this command, Shutdown Manager will start working based on the parameters you specified. The countdown will start and you'll see it on Main Window with big numbers:
You'll also notice than the other buttons will be disabled.
In the edge you'll see when Shutdown Manager expects to finish the countdown (Date in format Month/Day/Year) and also a progress bar.
You may cancel the Shutdown just by clicking again on the Shutdown button (or label), and the countdown must stop and disappear.
If you selected to Wait for a Window to be closed, then the countdown will not start when you choose Shutdown. It'll start only when the selected Window is closed. In this case, Shutdown Manager will notify you than is "Waiting...", it means, is waiting for the selected Window to be closed:
So, the "Estimated Completion" will show a Date/Time when the countdown will finish if the countdown start on that moment (based on how many seconds you specified).