You have a quoted one main use of a live cd.
On RIP live cd....you issue "exit' command to terminate the chroot environment and then and only then can you umount the relevant hard drive partition.
by defination, a person in chroot can not umount what is currently mounted....they can only work from within their new set of rules....the chroot environment knows nothing about the fact you mounted some partition under /mnt/hd or /zzzzzk.....it all looks like / to it.
To repeat you can not break out....except on a live cd...try exit.
2) what you are probably thinking about is chroot jails on hard drive systems