I believe there is a ball bearing under that black ring, and in the licked position, the ball sits in the grove on the PTO shaft. If it has been sitting awhile, it may have rusted and is binding. Spray it down with WD-40, and try and rotate or slide the black ring back and forth to see if it will loosen. When you get it off, take a wire brush and clean the PTO shaft really good. When you get it off, look inside the black ring and check the ball for free movement. You may need some assistance , someone pulling and you turning /sliding the black ring. Once you know how it works, it is easy.