(some days later, create second backup as delta from first)
$ wimcapture /some/directory bkup-2013-08-20.dwm \\
- --update-of=winbkup.wim:-1 --delta-from=winbkup.wim
+ --update-of bkup-base.wim:-1 --delta-from bkup-base.wim
(restoring the second backup)