This stitch is decorative and can be done over a base of blanket stitch or buttonhole stitch.
|Fig 1: Make a foundation of blanket stitch or buttonhole stitch. The illustration shows a row of buttonhole stitch.
Now, bring the needle out with a contrasting thread from the left end of the row, as shown.
|Fig 2: Whip around between each vertical stitch, as shown. Make sure not to pluck the fabric underneath.
A finished portion would look like this.