What he is talking about is doing the same a CSP does automatically, a controllable pitch propeller has to be done manually.
Fine Pitch to maximize rpm and coarsen the pitch to maintain rpm as speed increases.
If you don't lower the pitch, the propeller will begin to drive the engine and you will lose performance.
A given manifold pressure and rpm as listed in the POH will deliver the maximum performance for the condition flight listed. The pilot must maintain that rpm by controlling the pitch.
I have my pitch control set on a slider for the Bf-109 and adjust it constantly to maintain the desired rpm.
The basics of operating an controllable pitch propeller are given above.
It is all about maintaining optimum pitch and rpm.
As you coarsen the pitch to reacquire the rpm, you will notice an increase in performance.
In the quoted cases, they are setting the engine to a limited over boost and coarsening the pitch to maintain rpm.
That is how it works.
The experienced pilots you are quoting are coarsening pitch before rpm drops. If you know the approximate performance for a given setting, then coarsening the pitch at rpm will cause an increase in performance within limits.
It will also cause a subsequent manifold pressure drop and the performance will suffer as a result if outside of a narrow range.
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