Explain Draft On Tractors

Can someone give a good explanation of how the draft features work on tractors, and how to set them.  haven't ran one in 20 years since I was a boy and just don't remember. I gather that the AC tractors have a different system than others.

Basically in a nutshell, engineers know that pulling harder equals more tire slip, and less productivity.  So when you set the draft, you tell the tractor via linkages that if the pull force increases, lift the implement ever so slightly to put more weight on the rear tires, thus reducing slip.  Typically lifting the implement a very small amount will not reduce the implements effectiveness at doing whatever it is designed to do.  The reason this is automatic rather than just caring the weight on the tractor all the time, is to compensate for varying ground conditions, hills, and other things like compaction from the rear tires.  Every manufacture does it differently if they do it at all.  As Charlie described the way Ford used to do it, Allis did it with the lower drawbar connection, and on later tractors the point at which the lower 3-point arms connect to the differential case.