This variety of stitch gives a wavy effect to the pattern and follows the same stitching pattern as the Laced running stitch.

Back stitch: Lay the foundation by doing a back stitch

threaded back stitch1        threaded back stitch2
Fig 1: Take another thread and needle and bring it out through A. Pass the needle under the stitch A-B, from above, as shown. Do not pluck the fabric underneath.   Fig 2: Now, pass the needle under the stitch B-C, from below, as shown. Again, do this without plucking the fabric underneath.

threaded back stitch3   Fig 3: Continue this process of taking the needle under each back stitch from above and below alternately.

Fig 4: The completed pattern would show up like this.

Make sure that while taking the needle and thread under each running stitch, you do not pluck the fabric underneath. You can use a blunt needle to acheive this.
Make sure to leave the back stitch just a bit loose in order to allow the other thread to pass through it easily