Well in fact that's quite simple to use... Imagine that your other system, a redhat for example is mounted as /mnt/redhat... if you do /mnt/redhat/usr/bin/mozilla, it may complain 'cause it does not find the suitable libraries or things like that... Well simply do:
... Now in your shell where you have ran chroot, the new root dir is not / but /mnt/redhat, so if you do "mozilla" it will exec mozilla from your non booted OS and should run fine
Hope I'm clear enough
Oliv'