Binding Off Purlwise
Binding off purlwise is often done when you need to bind off on a wrong-side row in stockinette or a right-side row in reverse stockinette. It produces a bind-off that fits with the established fabric of your knitting.
To bind off purlwise, you simply work the standard bind-off with purls instead of knits. In other words, you purl 2 stitches, *pass the first stitch on the right-hand needle over the second stitch and off your needle, purl 1 stitch, repeat from * until you have one stitch remaining on your right-hand needle and none on your left-hand needle. Cut the yarn according to the pattern, draw the tail through the remaining stitch, and pull snug.
This tutorial demonstrates binding off purlwise. The explanation starts at 0:43, and the actual demonstration begins at 2:07.