I used to work at a facility that salvaged and rebuilt old air compressors for industrial applications. One of the salvage procedures required an operator to use a pneumatic torque wrench to pull a crankshaft out of each compressor that came down his/her line. Some crankshafts were more difficult to get out than others and required more torque from the wrench to remove. These crankshafts induced considerable counter-torque in the wrench, sometimes leading to the wrench being twisted out of the operator's hands. This was obviously a huge safety risk.
This fixture was designed to be attached to that wrench via threaded holes at the top and would stifle the rotation induced by the counter-torque of the wrench. The cut out section on the long side of this fixture fits around a gear on the crankshaft, and then the pin extending from the bottom of the fixture is lowered to rest against the side of the air compressor casing. When the wrench is activated, the pin, now constrained by the compressor casing, resists the rotation.
This was the first project that I ever designed and built all by myself, with the guidance of friendly and generous expert machinists.