An employee was directing a rear-discharge ready-mix concrete truck that was reversing so its discharge chute could reach the receiving hopper located at the rear of a stationary pump truck. As the employee was adjusting the chute on the concrete ready-mix truck, the ready-mix truck rolled forward, then backed up, pinning the employee between a pipe connected to the pump truck hopper and the ready-mix truck's DOT bumper. The employee sustained a leg hematoma that required surgery.