Try to do this ...
to make your knot look like this

A sheep shank has two uses. First, you may use it to shorten a rope and maintain its strength for pulling. Its most useful purpose, however, is in situation when you cannot retrieve your rope. When lowering yourself over a rock (with no intention of returning up the face) how are you going to retrieve the rope that it tied to a rock or tree at the top? Tie a sheep shank close to the top end of the rope. Maintain tension on the rope and clip the center strand inside of the knot. Lower yourself. At the bottom, give the rope a flip and the sheep shank comes loose, leaving only a short piece at the top.