Two crewmembers were working together to remove the Allen head bolts that secure the drawworks brake flanges to the drum. The drum had been removed from the skid and secured on the rotary table. The tools selected for the job were a ¾” drive impact wrench and socket, and a standard 9/16” Allen key. The tools were fitted up and the impact wrench was engaged. The tapered fit friction of the Allen head end of the bolt allowed the nut to break initially without applying torque against the Allen key. The impact wrench operator continued and as the nut loosened, friction lock was lost and the torque was transferred to the Allen key because lock nut was still bound to the bolt. The Allen key shaft length was a standard size and not long enough to react against the drum. The torque now being applied through the bolt to the Allen key was more than the other employee could hold. The Allen key slipped from his hand spun 3 or 4 times and was thrown free. The Allen key struck the injured employee in the forehead and above the upper lip requiring stitches in both places.