Ground layering for better root spread

In a recent post named Chinese Elm Broom, the author stated that he wanted to improve the nebari using ground layering techniques. In general, how is this done? I've ground layered a Japanese Maple to create a whole new root structure but not to improve a nearly mature one. I have a Zelkova with a nearly identical looking base as the Elm so could put this knowledge to good use.

Thanks
Steve

check out:
http://www.bonsai4me.com/AdvTech/ATNebari.html