An employee was working from a scaffold over a blowout preventer, using a 5-foot bar to position an orbit valve. An air winch on the rig floor was being used to lift the valve and hold it in place. The air winch line went taut suddenly and struck the 5-foot bar, pushing it toward the substructure of the drilling rig. When the bar struck the substructure, the employee's left ring fingertip was caught between them. The fingertip was amputated.