More of the Same
I really cannot see what is so difficult about this increased pressure business.
In a hover OGE air flows freely downward slowly dissipating as it goes, ultimately spreading out and going back up to replace the air which is still being drawn down thro the rotor. This takes the form of a very large doughnut vortex. We all know about the strong bit at the rotor tip but in a prolonged steady state this vortex has a reducing effect out to some considerable distance.
Now closer to the ground the downward flow is stopped at the surface. In order to stop motion a force must be present, or in this case a pressure increase. This pressure resists the downward flow of air so reducing the induced flow. For a given angle of attack/lift, this means lower pitch, more vertical lift vector and reduced rotor drag.
It is true to say the pressure under the rotor does not add lift, so NL is right in saying that. But the increased pressure is the mechanism by which the induced flow is reduced so it does reduce the power required.
Any lift produced by a heavier than air machine is ultimately supported by a higher pressure on the ground surface. Remember the old school days question....
A van weighing 1 ton has 1 ton of budgies sat on perches in the back. It weighs 2 ton. All the budgies takeoff and fly around inside the van, how much does the van weigh now?
Yes 2 ton! The birds are now supporting their weight with air which must in turn be supported by what is underneath.
The difference between OGE and IGE is only how localised the increased surface pressure is. OGE the increased pressure is spread over a large area and hence has little feedback, IGE on the otherhand the effect is very localised and hence has a larger return effect.
I am sure this is really quite a simple process and hope it seems so to you all. If I am missing something let me know.