The Prolong was a large rope that had a toggle on one end, and a iron ring on the other. it was normally carried on the gun trail, secured by the Prolong hooks.
When "firing by Prolong" the toggle end was inserted throught the hole in the lunette of the gun trail,the other end with the ring was attatched to the pintle hook on the Limber.
This allowed the gun to be dragged by the limber, and still be serviced by the cannoneers on the move. There was a specific drill used to load and fire the gun as it was being dragged along that differed somewhat from the usual procedure, as it was very difficult to prepare the gun for firing as it was being jerked over the ground.
When the piece was ready to fire, the drivers on the limber would stop long enough so that the gun could be sighted and fired, then they would resume their retreat (or advanceing to the rear movement).