OK, now I can't wait to get home and get behind my pride and joy and grab the pto shaft and test it the way your explaining.An overrun clutch allows for the PTO to slow without slowing the implement or damaging the PTO.
With the tractor off and the PTO engaged try and twist the PTO on your tractor it should slip or click in one direction.
An overrun saves abuse to the tractors PTO from a sudden shut off of the PTO from the tractor side.
A slip clutch or shear pin will protect the PTO in the tractor from sudden stopping / binding / heavy lag of an implement.
Shear pins are great for sudden obstructions, and slip clutches are better for over loading.
Now if it turns, that's my test to check it has an internal slip coupler ? Please bare with me on this..... Thank so much for your time.