Ring cables look like they are joining up at top and bottom by combining increases and decreases with cables worked at a steep rate and on wrong side rows. To start one off, you use a Christmas Tree Increase, either on the front of the work, or more likely on the back.
I first saw this method of decreasing in the Third Treasury of Knitted Patterns by Barbara G Walker, which has a rich chapter of such designs.
Capping a Cable (Also known as Decrease
5 (7) stitches down to 1 in Barbara G
Walker’s Closed Ring Cables)
Slip 2 (3) stitches onto the right needle
purlwise.
* Pass the second stitch from
the tip of the left needle off over the
central stitch.
Pass the central stitch to
the right needle.
Pass the second stitch
from the tip of the right needle off over
the central stitch.
Pass the central stitch
back onto the left needle* Repeat * to *
1 (2) times. Purl the central stitch unless
otherwise noted.
There are more tutorials!
Tree Increase
Tree Increase from the back of your work
Capping a cable
Half-Bauble beginning
Half-bauble ending