Couching is simply sewing a heavy thread in place with a finer thread. Its a retro technique, too, that was originally done by hand, but it works beautifully by machine as well. I use a zigzag stitch to secure the heavy thread in place.
Love the comment from other pinner, "How to use a double needle tutorial.shut up! This is amazing. I learn new sewing stuff all the time!" The other thing I didn't know is the double needle seam stretches.