Letsby Avenue,
Instead of lying to them about pressure below the rotor, why don't we tell them there is a magnet in the transmission that repels the earth, and is worth 15% power when you get close? It fits, everyone knows that grass isn't magnetic, so ground effect is weaker over grass!
How about we tell them that there is a big invisible elephant below the aircraft, and we have to squash it wnem we land, and elephant flesh is particularly tough, so it takes 15% of the weight of the aircraft to squash it!
We could lie about everything, to make it easier to learn! The max rotor rpm should be observed, because the rotor remembers each rotation, and it gets pissed if we eat its life too quickly!
Why don't we ppruners invent better ways to explain things, instead of that silly, hard to understand truth!