Threaded backstitch (also called laced backstitch) is a hand embroidery stitch made combining the foundation stitch (backstitch) and threading.
After you embroider the backstitch, lace the second thread through the backstitch without entering the fabric or crossing the backstitch.
You can embroider the threaded backstitch in a single color and create a wavy line or choose two contrasting colors to create exciting patterns. Keep in mind that backstitch will be more visible than threading.
Watch this video tutorial and learn how to embroider a threaded backstitch.