EE WAS RETURNING FROM DUMPING A LOAD OF ASH. HE FAILED TO LOWER HIS BOX & AS HE PASSED UNDER #1 CONVEYOR THE FRONT OF HIS BOX (CANOPY) CONTACTED THE UNDERSIDE OF #1 CONVEYOR CAUSING THE TRUCK TO TIP BACK, RAISING THE FRONT TIRES OFF THE GROUND. THE MOMENTUM OF THE TRUCK ALLOWED IT TO GO UNDER THE CONVEYOR AT WHICH TIME THE FRONT TIRES RETURNED TO THE GROUND JARRING THE EMPLOYEE.