First - find out what backup-file you do require in the so called list-file, e.g.:

bzip2 -dc system_www_2005-08-12_incr.list.bz | less

If you require only one specific file - extract it to a temporary directory - e.g.:

afio -i -Z -P bzip2 system_www_2005-08-12_incr.afio

Note - that you can get the options out of the regular backup-report you get:

*** Compression: yes (-Z -P bzip2 -Q -c)

If you add a supplementary -v -z - you'll also have some listing flying over your screen.