Dave M. has the definitive answer in his message 8/27.
I have a Ford NAA (Jubilee).. what Dave described as a 'transmission driven' PTO.
My brother has a Ford 801 with live PTO, what Dave describes as a '2 stage clutch' .
The most modern live PTO is as Dave describes as 'INDEPENDENT'.
At issue is SAFETY !
With OUT a live PTO, pressing in on the clutch will disengage the tractor drive transmission but the PTO, connected to a bush hog still spinning, will transfer this power to the tractor drive transmission continuing to push the tractor forward. "BAD' using Dave's description if you are about to run into the side of a barn and can't stop.
One remedy ( I'd rate it better than BAD not quite as good as the 2-stage clutch)is to install an OVERIDING CLUTCH. This is installed between the PTO and BUSH HOG (or mower etc) and
essentially freewheels the bush hog from the tractor drive transmission.
Best link I've seen showing an over riding clutch connection to a Ford Jubilee is on Ron Webers web site at