Undo a change
Undo writes back the exact previous value and reads it back to confirm. It works at any time, even after the change has been scored.
Every applied change stores what the field held before. Undo writes that value back, exactly, and reads it back to confirm.
Undo works at any time, including after the change has been scored. The result stays in your history so the same kind of change is not proposed again for that page soon.
If the plugin on your site is disconnected, undo waits until it is reconnected.