Basically, you are using Kitchener stitch to graft off the stitches. In order to do this, you must first separate the knit stitches onto one needle and the purl stitches onto another needle. This can be done on 1 by 1 rib or 2 by 2 rib very easily. You simply divide your sock stitches in half for front and back. Then begin with the front stitches on one needle and divide them onto two needles by slipping the knit stitches onto another dpn and by slipping the purl stitches onto a third dpn as you come to them. You will eventually have two rows of stitches, the knits in front and the purls in back, just like you do for Kitchener stitch when you close of the top of the toe in top-to-toe socks. When you get to the end of the first set of two needles, you will need to separate the knits from the purls for the back of the sock just as you did for the front.
Here are some illustrations to help you see what I mean. This first photo illustrates the way the knits and purls are separated on two needles and shows the first step in which you use a tapestry needle to enter the knit stitch as if to knit and then take it off the needle as in the first step of Kitchener stitch:
This next photo illustrates the way you use the tapestry needles to go into the second knit stitch on the front needle as if to purl and then you leave it on the needle.
Now, you bring the tapestry needle to the back row of purl stitches and enter the first purl stitch as if to purl and take it off the needle:
The last step is to enter the next purl stitch on the back needle as if to knit and then leave it one the needle:
I found as I always do with Kitchener stitch that I needed to sit in a quiet corner for 15 minutes and really concentrate. It is slower than a normal bind-off, but well worth the effort since it is stretchy while being neat and tidy. Here is a photo of the finished graft-off edge (note the sock pattern included a bit of ribbing at that top so it makes it easy to use this technique; if your pattern doesn't, you might want to add it.):
There are so many ways of doing things in knitting, so I encourage you to try different techniques. After trying a few of the different stretchy cast-offs, I have seen, this is definitely my favorite. Please share your favorites in the comments section.