Jails and fstab/mount

[Axel S. Gruner]

Hi.

I saw a new "feature" for Jails an wonder how to use them.    
/etc/default/rc.conf:
jail_example_mount_enable="NO"
jail_example_fstab=""

Ok, any jail can have an own fstab (/etc/fstab.myjail) and mount und
unmount the fs in the jail fstab inside the jail? 
Where can i get some information how to use these features and what are
these features good for? 

Thanks in advance.

asg