The Plaited Edge Stitch is a kind of Buttonhole stitch, that in looks can be mistaken for #66 Berwick Stitch.
As the name implies it is useful for edging, either on a folded hemline or a raw edge. The closer together you place the stitches the less the fabric is at risk of fraying.
I have used the stitches as fancy embroidery stitches, not for hemming.
Instructions can be found in these two books: Mary Thomas's Dictionary of Embroidery Stitches and A-Z of Embroidery Stitches 2.
Here are the steps in my photo guide:
Take a straight (back) stitch and exit
to the left of where you started.
Insert the needle under the Straight Stitch
(without biting the fabric) and pull tight.
You now have a wrap that looks almost like
a knot.
Continue in the same way by inserting the
needle diagonally.
Anchor the last stitch.
The stitches sit nicely, but are a bother to unpick, so pay attention to tension while you stitch.
On a folded hem they look like this:
Homework: