To open it, click on the light blue button with the reversed cirular arrow around a clock:
Then you can see the underlying history, e.g. here for exactly this
time-travel.rst file of the documentation in front of you.
- (1) is the control knob of the slider
- (2) to view changes between two revisions
- (3) to revert the file back to the given revision. This doesn’t undo the history, but adds a new entry at the end. No changes get lost.
For very long histories, another button will show up to load the entire history.
[x] Changes enabled, you get a two-ended slider to see the changes between these two revisions:
For more information, read this blog post about time travel.