something is wrong here .
there is no "idle" on the prop lever- you set the governed prop rpm or- pulling fully back , feather the prop.
the condition lever is the whole time at "idle" ( in some application low or hi idle) . pulling it fully back means shutting the turbine.
like written above most probably he pulled the prop back to feather without first pulling the power back and screwed the gearbox .
the governor has very little in common with thin air - and feather the prop on a really idling engine is not an event for the pt6 since its a free turbine where the power section is not mechanically connected to the gas generator .
cheers