When should User Interface Preservation kick in?

I am trying to get User Interface Preservation working but without luck.

When will the encode functions be called?

I've set isRestorable, restorationClass and identifier properties in my Window but window:willEncodeRestorableState: is never called.

I am expecting it to be called when I quit my application but it is not. Do I have the wrong expectation, or am I doing something wrong? I've followed the steps under "User Interface Preservation" and also this tutorial.