The injured employee backed a counterweight truck up to a crane to unload and position crane mats and install counterweight. The crane raised its boom off the boom dolly and swung to the right to erect the swing away jib. Employees then hooked up the overhaul ball to the load line. The crane then boomed up and centered over the rear of the crane carrier. The brakes of the boom dolly released and the unit rolled forward pinching the injured employee against the rear of the crane. The injured employee sustained internal bleeding in their abdomen and an abdominal tear.