This neat device was developed by the Shuttle Brothers Randy and Gary Houtz. For a full explanation and patterns see their book:
"Tatting the GR-8 Self-Closing Mock Ring" (available from The Shuttle Brothers, Roseground Supplies or Lyn Morton - see Links)

The SCMR is actually a modified chain, but the important thing is that it can be used to make rings that come off rings, not split-ring style, but in the way that we make rings coming off a chain.

Using ball and shuttle

1. For practice begin by making an ordinary chain, just a few ds to get started.
2. Then pull down a loop of shuttle thread (large enough for the shuttle to get through), pinching it between left thumb and finger next to last stitch made.
3. Next make the chain that will be the SCMR, making sure you keep the loop. When you've done the ds for the mock ring, pop your shuttle through the loop and pull to close ring. Then carry on with chain.

Using 2 shuttles

Just as we make rings coming off chains by switching shuttles to use the 'ball' shuttle as a 'ring' shuttle, we can make rings coming off the SCMR. Try it out with these variations on the 6 ring/chain motif:

Use 2 shuttles with continuous thread, Sh1 for central rings and Sh2 for chains and SCMR. Use Sh2 for rings at points:

R: 4-4-4-4 RW
*Ch: 4-4 SCMR [Ch 6 switch shuttles
R: 4-4 switch shuttles Ch 6]
(']' means you put shuttle through loop and close SCMR)
Ch: 4-4 RW
R: 4+4-4-4 RW
Rep from * until 6 points complete, joining last central R to first. Tie ends through first ring.

After doing the snowflake above it is easy to do this variation with more rings coming from each SCMR ring:

Use 2 shuttles and continuous thread, here Altin Basak.
R: 4-4-4-4 RW
*Ch: 4-4
SCMR [Ch2 switch shuttles, R8, switch shuttles ch2…. until you have 5 rings, ch2]
Ch: 4-4 RW
R:4+4-4-4
Rep from * until 6 points complete, joining last central R to first. Tie ends through first ring.