AFAIK the crucial part of this, is rsync's `--link-dest=DIR` parameter, which hard-links an unmodified file to the respective place in "DIR", rather than making a copy.
The docs for my version of rsnapshot say not to use '--link-dest' if GNU cp is available, because the script will use 'cp -l' instead when copying "daily-1" to "daily-0" (for example). This is before it will overwrite "daily-0" with new files via rsync.
