A motor drive shaft assembly. If this little gear geats stretched out it can cause the windlass to bind. It's almost impossible to really tell if it is stretched out though. It works in conjunction with the clutch bearing in the motor. It is the most common problem with these windlasses. also if the clutch bearing in the end of the motor is shot, this too can cause binding. The bummer is that the little gear assembly costs a frigging $132.10 !!! got any doubts, ship the windlass to me. I repair them for a living