As far as I am aware, in your example, the pulling the pitch back and watching the red line increase is a gearbox limit.
SHP is worked out by Tq X RPM divided by 5252. So in reducing the RPM, you can increase the torque and produce the same SHP.
Hence the two red lines on the steam gauge vans and that little table that is printed on the panel in front of you.
I think the first three figures all give the same amount of HP (within 1 or 2hp).
I'm getting the funny feeling that this is one of those "Never operate lean of peak" PoH things.....
Maybe I will take this over to the tech forum and see if I have any joy there!