Looked in the parts book for my 1953 Mustang, The spining blades would push the tractor along as you said. I guess it don't happen with the finish mower because it has 3 smaller blades rather that the one huge brush hog blade .
Not as much force made with the 3 blades.
Now I need to look at a live power set up. My 5000 Ford does have clutches, just not sure how they work.