The extension key: how do they work? Do they work only if the trial license has expired?

Trial extension keys work with the trial expired and with not expired trial.

In case that it's not expired, it extends the current trial status. For example, if you create a trial extension key with 10 days trial then:

1) If your current trial is expired, you will get 10 days trial left.

2) If you currently have 5 days left, you will get 15 days left with the trial extension.

Notice that you can ONLY extend the trial settings that you selected in the "Trial Settings" panel in WinLicense before protecting your application.