At the back of the whipple-tree there are two "revolving shafts," extending from near the middle to the ends; the ends of these shafts are bent at right angles, and they are so arranged that the outer end of each one, which is flattened for that purpose, covers the end of the pin on the whipple-tree, to prevent the traces from flying off; and to the inner ends a cord is attached, to enable the driver to turn the shafts, and thus uncover the ends of the pins, and permit the traces to escape.