The View Changes Since Backup option will allow you to compare the current files on the system against a backup of your choice.
When you first click on View Changes Since Backup you will see a screen similar to Create a New Backup. Select which named backup your backup is located under, then hit Select. You will be viewing a list of all the backups you have made in this named backup.
After choosing which one you want to use to compare with hit the Diff Backup button. Because this could possibly put a heavy load on the system you are asked to confirm your decision.
Once you hit Really Compare the process will begin. Upon completion you will see a summary screen, similar to when you create a successful backup, listing all the changed files.