One dot right, you are on centreline.
In long range cruise on R2800 there were 2 leaning techniques, "2 bmep drop" and "11 bmep drop". Couple of times a dozey F/E would go past 11 bmep on the lean side, leading to a sudden drop in bmep to almost zero. Prop kept turning same RPM, MAP stayed same unless throttle moved. Capt blood pressure rose. Engineer bought many beers.
I suspect if airspeed low enough, prop would not have enough "fine pitch" available, and RPM would start to decay with decaying airspeed. But that area of flight is academic and of interest only to the very brave.
Use of feathering pump became very apparent when I had a runaway prop on DC-6B. Prop governor(which normally controls RPM ) mechanically broke (governor control spring failure) resulting in loud howl from prop and Capt simultaneously.
Luckily, feathering pump pressurises governor control piston to port oil to "increase pitch" side of prop servo piston. It seemed like an age, but prop eventually slowed a bit, and then feathered normally. Ten more grey hairs.