In this increase a new stitch is created by working under the bottom strand of the horizontal chain between the stitch just worked and the next stitch. It works best in a row with Tunisian reverse stitches:
It does look a bit awkward, but this is how it is done.
Insert your hook from back to front under the top strand AND the back ridge of the horizontal chain before the next stitch (so that the hook is between the top and the bottom strands):
Grab the bottom strand by rotating the hook away from you (as you see the bottom strand is now on the top of the hook). Yarn over and pull up a loop:
Leave the loop on the hook: