The engineer was repairing the Top Drive System (TDS). To do so, the crane was hooked on to the TDS swivel and lifting attempts were made to disconnect the quill from the drive gear without success due to some obstructions. At this time the TDS engineer went up on the stand to see the obstruction and he disconnected the crane hook from the swivel, leaving the swivel link in the vertical position. Whilst inspecting the point of obstruction, the swivel tilted to the horizontal position trapping the engineers left arm between the swivel link and the TDS stand resulting in a fracture hand.