The operation was Pulling Out Of the Hole (POOH) with 5-1/2 inch drill pipe. The Derrickman was pulling back the stands of drill pipe with the assistance of an air hoist with a chain and hook attached. After setting down stand # 26 in the set back area, the Derrickman unlatched the elevators and the Driller retracted the link tilt. The Derrickman then used the derrick mounted tugger (air winch) to pull the stand of drill pipe back towards the finger board, clear of the traveling block. The Driller commenced lowering the traveling blocks and after approximately a ten foot descent, the Top Drive came into contact with the top of stand #26. As the Top Drive impacted the stand of pipe, energy was transferred causing the pipe to bow and snap back towards the Derrickman who was positioned on the monkey board. The stand struck the Derrickman knocking him backwards causing his head to strike a derrick beam. Although no one actually saw the incident take place, it appears that the Derrickman lost control of the pipe thereby allowing it to fall back in to the path of the descending top drive.