There is a very specific way to make a double-back knot. If one is right handed, the second loop must be to the left of the first loop—and the free end passes from left to right through the loops. The second loop is used to tighten the first loop, and bring it close to the last bead. Then, the free end is used to close the second loop. Since the Chinese were using two cords, it is possible to divide the cords, and pull them apart to further tighten the knot.
If these steps are not taken, the knot can be a disaster.