Aaaaah - overpitching - I wondered when this would raise it's ugly head! Overpitching is simply when you top out the N1/Ng whatever of a gas turbine (merely using gas turbine because most are familiar with terminology - happens in pistons as well) - in other words engine can give no more but you as a pilot keep pulling lever - drag on rotor system increases - blades slow down - lift is lost.
GAGS,
E86
PS I trained through the US Defence system - aerodynamics was over simplified to suit the "sausage machine" process. It wasn't 'til I attended the "School" that I gained a clearer understanding of helo aerody.