You know, Jeff, I got to thinking.
The little flap that controls where the airflow goes to has a foamy rubber gasket around each end. When you hit the notch, the motor to the door seals the passage to prevent any air from coming from anywhere but the core or evaporator, as the case may be. This could cause increased pressure in the ducts, since the blower has to suck -only- from the passage. When you turn the control off the notch, the seal might be broken just enough to allow the blower to openly suck from the now broken seal, hence taking a strain off the blower and, in effect, drawing less power.
Same principle as a vacuum cleaner running, then putting your hand over the hose. Higher motor stress, more power needed, a slight demand on the alt. Just a theory.
As far as a diagram, not really. But ask what you need to know and I can probably photo/walk you through it. I am usually given cars with vacuum problems, for some reason, at the shop.