An employee and a co-worker were loosening nuts and bolts on a 24-inch, 300-pound flange with hydraulic torque equipment and a reaction arm. The hydraulic torque equipment was not functioning due to pump issues. The co-worker went to inspect it when the pump kicked on and engaged the torquing equipment. The torquing equipment moved the reaction arm, which struck the employee's right-hand index finger, resulting in a fingertip amputation.