A driver was uncoupling her tractor trailer combination and was having difficulty pulling the king pin, release arm. The injured employee went out to assist. He stepped between the tandem axle drive tires on the power unit and reached in to grab the king pin, release the arm and pull it back to release the locking pin. Once he had the arm secured and the pin released, he instructed the driver to pull forward. As the driver pulled forward, the injured employee was struck on the right leg by the rear axle tires. The injured employee fell to the ground and sustained a compound fracture to the lower right leg, ankle, and foot.