This stitch is decorative and can be done over a base of blanket stitch or buttonhole stitch.

You need to know the blanket stitch or the buttonhole stitch to be able to continue with this tutorial.

whipped_buttonhole_stitch_1       whipped_buttonhole_stitch_2
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.