Paradism... ! I didn't want to be pedantic, but perhaps I needed to be.
Let me try to explain (this is the Tech Log forum, after all):
1) The rise and fall of the engine note that they are talking about is at the beginning of the take-off run, as the throttles are being advanced, not when the aeroplane is getting airborne.
2) On the Dart, the propeller is in ground-fine pitch initially (for taxying without overheating the engines, and to provide engine braking if the throttles were closed).This effectively stops the propeller coarsening its pitch enough to provide the thrust necessary for flight.
3) As the power comes on, the propeller spool and propeller therefore accelerate rapidly. At some point, the system recognises that take-off power is required, and removes the ground-fine-pitch stop. The pitch now coarsens suddenly, momentarily slowing the propeller spool (shaft) of the engine down until the rapidly-increasing power from the core spool wins the contest. [The Dart is a 2-spool engine.] From then on, the propeller continues to accelerate, even though its pitch may continue to coarsen.
4) The transition from ground-fine pitch to flight pitch - and vice-versa - is one of the key aspects that turbo-prop crews have to monitor, certainly on the Dart engine.
5) It is even more important on landing. On the Dart Herald, for example, a lever had to be pulled back when the aircraft was safely on the ground. This engaged ground-fine pitch, resulting in a momentary increase in propeller rpm and useful drag to slow you down. The Viscount may be slightly different, but the end product would be the same.
6) NB: Woe betide any Dart crew which fails to ensure that the props are in ground-fine pitch for taxying after landing. If not, opening the throttles on the taxiway may result in the TGT (turbine gas temp) going off the clock, and melted turbine blades.
Phew!
The sudden coarsening of propeller pitch in (3), above, is the cause of the rise and fall of the engine note, referred to by jh5speed. Lads who inhabit the perimeter fences at airports all over the world are more than familiar with the Doppler effect. I should know - having been one of them myself. So I've heard the Dart from the perimeter fence and the cockpit.
Hope you are now convinced!
PS: The Dart sounded great, but what about the whispering Proteus...