what does the --backup-mode option for rdiff mean?

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http://serverfault.com – I've got rdiff-backup up & running nicely using cron jobs over SSH between two VMs (my webserver and a backup space), using scripts put together from various places online. I'm backing up various folders, including database dumps, /etc and /var/www. I've noticed that the script uses the --backup-mode for the webroot folder, but not for the others. Can anyone explain what this option does? All I can find online is the man page which doesn't shed much light on things: Force backup mode even if first argument appears to be an increment or mirror file. I can't find any mention of what 'ba (HowTos)