I found this in the Slackware Linux Essentials book for Slackware 8.0 that came with Slackware 9.0:
We call the main directory the root directory, and it's denoted with a single slash (/). This concept may seem strange, but it actually makes life easy for you when you want to add more space. For example, let's say you run out of space on the drive that has /home on it. Most people install Slackware and make one big root drive. Well, since a partition can be mounted to any directory, you can simply go to the store and pick up a new hard drive and mount it to /home. You've now “grafted” on some more space to your system. And all without having to move many things around.
(Note: I am not doing this. I am just trying to understand it.)
I don't see how expanding /home with a new drive is accomplished with this 'grafting' they are talking about. Is there some symbolic link mechanism that appends the new 'space' to the /home directory?