The rotary cutter should have come with an Owner's Manual that shows how to do it, but the online version of the manual shows the Model 12 to have the pin type release. With everything shut down, feel the very front end of the PTO driveshaft at the tractor and you'll find a spring loaded pin that you can press in. Press in the pin and hold it in while you pull the shaft loose from the tractor. When you hook it back up, you can slide it partway onto the tractor output shaft, then hold the pin in while you slide it all the way forward; release the pin and pull back to let it click into place.