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So after doing some research - the short answer is yes: the HPC levers should be moved as you climb out (blue lights off) and moved back in on approach (blue lights on).
Any time you are going to fly at low speeds or taxiing on the ground the propellors should be in Lock-Out (blue lights on).
The longer answer is this (and perhaps someone can verify this):
The propellers on the Dart engine can rotate between 0° (ground fine - flat against the airflow. Also the start position) to 87° (feathered - almost parallel to the fuselage, in line with the airflow).
When in cruise and flying at high speed it would be disastrous if there was a mechanical failure and the propeller blade rotated toward the 0° end of the range creating drag instead of thrust. For that reason a Pitch Lock is engaged when the HPC is forward of the “Open” position preventing the propeller blades from moving below 32°.
When on the ground or flying at low speed a propeller attempting to spin using 32° pitch or more would create drag on the rotation of the engine, slowing its RPM, if you then increase the throttle in that state - the air to fuel ratio will be unbalanced and the engine will quickly overheat and burn out. The Lock-Out allows the propeller blades to travel below 32° and prevent drag on the rotation of the engine.
Probably the best explanation of this system, I have found, is in this crash report here:
https://assets.publishing.service.gov.uk/media/5422ef5f40f0b6134200023b/dft_avsafety_pdf_500083.pdf
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