After reading about guys using a length of chain top link, I made one. I considered what happens if the mower or land plane tips up all the way. My rotary mower would tip up to about 60° before the A-frame hits the back of the tractor diff. housing, but by then the PTO shaft has already broken, which I should've already known and shut down the tractor. Using a short pipe 6" shorter over the chain prevents this from happening.
I wonder how many tractor operators don't watch the implements they are using, just turn the key and steer? This can cause problems though, I have a loose disc in my back from looking over my right shoulder watching what's happening; my chiropractor fixes me about 1-2x/week.