The Stein knot (aka Stone knot) is a variation of the Figure 8 knot. It is used to secure a rope that is already passed around a post or through a ring. It is quick and easy to tie and untie. It is a device rigging rather than a true knot.
In canyoneering, it is used to isolate rope strands to allow one person to rappel while another is getting on the rappel, or allow rappellers the option of using a single or a double rope.
It is documented in the online book "Life on a Line" by Dr D Merchant
http://www.draftlight.net/lifeonaline/download/download.php?section=1 (pdf file, 1.04 Mb)
See Chapter 4: 17 essential rescue knots. Page 38