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.
Impara a cucire a macchina: un corso di cucito gratuito per prinicipianti
Sewing lesson: How to sew a buttonhole with or without a buttonhole foot on a sewing machine
Come fare lo sbieco con lo sbiecatore - Cucicucicoo