THE EMPLOYEE WAS OPERATING A Z6T HAUL TRUCK WHILE CLIMBING A 12% GRADE AND NEGOTIATING A LEFT-HAND RADIUS CURVE. THE EMPLOYEE STATED THAT HE LOOKED BEHIND BECAUSE HE THOUGHT HE SAW A LIGHT. HE ALSO HAD HIS LEFT ARM OUTSIDE OF THE OPERATOR'S COMPARTMENT. THE TRUCK CRUSHED HIS LOWER ARM AGAINST THE RIB. HIS ARM WAS AMPUTATED AT THE HOSPITAL, BELOW HIS ELBOW.