I understand the 'load' part.
Back in the late 80's, I owned a Kawasaki GPZ750 turbo (what an animal!) and the boost gauge rarely got up to full boost unless I gave it 'loads' of throttle in a higher gear.
Keeping it at or near the rpm red line resulted in lowish readings on the boost gauge, not that I could look at it too much at those speeds, as it was mounted on the tank.
But I think I've got it now. Put simply, at lower rpm, the engine isn't sucking as hard so the pressure in the inlet manifold goes up, and with no alteration to fuel/air metering would result in a dangerously weak mixture.
Conversely, increasing rpm makes the engine suck harder, lowering the pressure in the manifold, and without alteration of fuel/air metering would result in an overly rich mixture.
Have I got it?
Thanks for all the posts if so!