Incremental help

[Banana Flex]

hi all

i'm playing with rsync to make a good incremental backup on Mac OS X  
and FreeBSD
i use this with the --delete-excluded option to get a fresh copy with  
only modified files

but today i ave a new thing:
i would like to get a backup folder with all modified files on it,  
not the full backup, only modified files
i'll tried -b --backup-dir but isn't working

can any help me configure rsync to get this works?

thanks a lot

Banana