I found some info on Reddit Engineering Porn
"My family has a similar design for the chain link fence gate around their pool. The bottom hinge operates very much like this.
The top hinge is simply a vertical static pin mounted to the fence and a collar mounted to the gate that slips over the pin, very much like a standard hinge.
The collar is just loose enough to allow the axis to precess depending which way the gate is opened. Quite a simple design, but extremely effective."
It stays on he plate because
"The "slots" are smaller than the diameter of the pin. The pin is cut into a D shape so as the gate closed down on it the pin slips freely into the slot
then as it turns away the flat spot rolls inside of the hole at the end of the slot and it is too large to go out of the slot."
aha! I found it by googling for "rising gate hinge"
Good call. Did some googling and it sounds like it allows the gate to “rise” to accommodate an incline - like if it’s in front of a driveway, swings inward to the driveway,
and the driveway slopes up. I guess the gate is forced upwards a bit when it swings? Not sure, but pretty cool find.
Looking again at the photo you just posted Lee, I can see that the gate is held at an angle which looks to me like it swings in an arch and coming at a balance point in the middle of the swing which happens to be in the closed position. Nifty idea if this is correct.... No more worries about whether the gate is closed (but still unlatched) or not.