i personally very much doubt beta range ( so lifting the power levers over the gate) inflight is approved for any pt6 equipped aircraft and is misunderstood with the procedure pulling back to flight idle which will result in torque dropping to zero and the beginning where the wind drives the prop in a dive. ( pt6a will allow this- on a pte331 the negative torque sensing would kick in ) . the prop governor in this situation is still active and may reduce blade angle further to prevent a prop overspeed in such a dive. the sound is noticeable when this begins.
going really into beta means the governor is ovverridden and prop pitch is controlled with the power lever ( before pulling back further and reverse kicks in- this is governor controlled again ) .
going beta inflight means beyond the really massive drag having no propspeed governor and nothing prevents you from a prop overspeed.
i do not know a porter in specifics, but like said i believe you misinterprete the sound and drag from closing power fully ( flight idle ) where the governor dictates a very flat pitch to not overspeed the prop which is driven by the wind in this situation ( the prop disc braking phenomena) and real beta range with prop governor disconnected.
you can clarify by asking a real porter pilot if he pulls back the power lever beyond flight idle by lifting it and going over the gate inflight- i do not think so.
cheers
ps : hyueman - are you from the same project like another person who contacted me regarding pt6a operation principles ?