I fell asleep last night trying to analyze this using my "simple machines" physics from 50 years ago. 🙂
I think that goes like this....
Instead of an Ollie, we have a sled with 4 wheels that spin and roll frictionlessly across a plane. If that plane is inclined, the sled will always roll down hill. To keep it from rolling, put chocks (another inclined plane going the other way) on the downhill side of the wheels. If held motionless, there will be force on those chocks equal to the force pushing the sled down the hill (sine of the angle of incline IIRC??? , doesn't really matter).
It's clear from this picture, the bigger the incline, the bigger the chock needed. (IOW, a little chock will just get run over before creating enough back pressure.)
It's also clear that, if you put a big enough chock on the downhill side, and drag the cart "up" it, eventually the cart will "roll backwards" when released.
So, how do you know when you are in the sweet spot? How do you know if you let go of the cart, it won't roll? How do you know if you have 0 tension on the hitch?
Honestly, I don't have an answer for this. Unless there is some way to actually measure pressure at the ball.
It leads me to conclude that I have been getting really lucky, and that I don't live on a frictionless plane. So far, friction has made my guesses "good enough".
Henceforward, I'm vowing to chock all 4 tires, front and back, before unhitching. I'm even considering the advice to leave the chains on.