OK, I'll try this:
An implement (plow, cultivator) on a 3-point hitch kinda pivots around the axis of the bottom 2 links. The pressure of the soil against the implement tries to swing it back and up. (Viewed from the left side of the tractor, this would be a counterclockwise rotation.) The upper part of the implement presses against the top link, which, through a spring, presses against a valve inside the tractor. When soil pushes against all this stuff hard enough to overcome the spring and open the valve, the hydraulics will raise the lift until the soil pressure drops enough for the spring to close the valve. In draft control, the idea here is: as the front wheels of the tractor start to go up a hill, the implement, being behind the rear wheels, is forced deeper into the soil. This extra pressure activates the mechanism and raises the implement, maintaining a constant depth.Position control overrides the spring and valve, and keeps the lift at whatever height you put it at. I'm sure someone else could do a better job of explaining this, but I hope it helps. Jon
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