What they said doesn't necessarily have anything to do with mirroring.
When we take a backup of MySQL to our backup server, we also create a my.cnf file configured to use said backup. As in, we have multiple config files, each created at time of backup, specifically referencing that individual backup. Each can be easily used to start MySQL up using that X day old backup. Then I can shut it down, and start it up again using a completely different backup just by specifying that backup's my.cnf file, all without modifying either backup.
This is also nice because it makes it simple to make sure your backup actually works.
When we take a backup of MySQL to our backup server, we also create a my.cnf file configured to use said backup. As in, we have multiple config files, each created at time of backup, specifically referencing that individual backup. Each can be easily used to start MySQL up using that X day old backup. Then I can shut it down, and start it up again using a completely different backup just by specifying that backup's my.cnf file, all without modifying either backup.
This is also nice because it makes it simple to make sure your backup actually works.