In the technique of underside couching, thread (often gold) is laid on the surface of the ground fabric, couching threads are then passed over it. As each couching stitch is worked over the gold thread, the needle is carefully re-inserted into the hole in the backing fabric that the needle created on the way out.
The couching thread is pulled tight and a tiny loop of the goldthread from the surface drops through the hole in the backing fabric to the underside (thus giving the technique its name).
This creates a hinge in the gold thread, allowing the fabric to bend and giving it a great flexibility. Fabric worked with gold thread in underside couching has much more drape than fabric with surface couched gold, thus making it a much better technique for working objects which will be worn, such as ecclesiastical vestments.