An employee was standing on a flatbed trailer, guiding a crane hook to move A-frames on the trailer. As the crane operator moved the hook upwards away from the A-frame, the hook became detached from the line. The hook assembly then fell from a height of approximately 12 feet, first hitting the steel A-frame, then hitting the employee on the back right side. He was knocked down onto the trailer, suffering a back injury and broken ribs and requiring hospitalization.