I first tried this technique on a volunteer American Elm seedling I had found growing in the yard. After a year or two of working on the root base, I transplanted it to a sunny growing ground and planted it on a ceramic tile. Three or four years later, it was about 20 feet tall. Since it cost me nothing, I added a tourniquet using a radiator hose clamp to ground layer a new set of roots and dug it the next year. Worked much better than expected, with 360 degrees of roots all along the same line. Used a reciprocating saw to cut the original roots off and it didn't miss a beat.
Last year I tried a similar technique on couple of zelkovas I was growing in the ground with 3-4" trunks. This time I did the ring method by stripping the bark, added root hormone, packed with sphagnum, covered with mulch, and dug them a couple of weeks ago. On one, I only had a couple of spots where I got new roots so I just kept the original roots (probably because the mulch layer got eroded and the layer area was exposed to the elements).
Second I had much better success with a ring of fine roots around about 80% of the trunk. Used the reciprocating saw to saw off the bottom original roots.
American Elms and Zelkova grow really quickly in the ground in my area. You can get 3+ inch trunk in as little as three years. Just have to watch out as the roots grow really quickly and they can be a pain to grow out. I highly recommend planting them on tiles after a few years of working with younger plants to remove tap roots and roots growing straight down. The tile helps with the root spread, and allows you much more easily dig them up. Even then, digging these was a lot of work and required dosages of beer and advil after.