You can check any two subtitle versions on the revisions list to compare them. By default, the last revision is always checked, so, if you need to compare two other versions, please uncheck the most recent one to be able to select something else.
After checking the selected revisions, click Compare Revisions button:
This will open a "diffing" page where the selected version are displayed side by side, with any differences in text or timing highlighted in red:
On this page, you can quickly switch to earlier or later versions in the left and right columns by clicking <-Previous and Next-> links. This helps you to easily track changes made in each particular version without needing to return to the revisions list.
If necessary, you can roll back to any selected revision from the diffing page by clicking the red Roll back to version button.