The Torque Limiter is not actually in the FCU; its a pressure operated valve normally mounted on the Reduction Gear Box just aft of the Prop Governor, and has the same ability as the Prop Governor to bleed-off Py pressure from the FCU (aka "Fuel Topping"). It receives the same input of Torque Oil Pressure as do the Torque Indicating system and the Autofeather/Auto Ignition systems (if fitted).
With excessive oil pressure (which is directly proportional to to prop torque) the Limiter bleeds-off Py pressure, which changes the balance between opposing air-pressure capsules in the FCU, serving to reduce fuel flow, and so torque. As torque reduces, the system modulates until such time as power demand is reduced.
Like all the clever stuff on the PT6 its completely pneumo-hydro-mechanical, so doesn't need electrical power.
CAUTION: A bit like auto-land; not all aircraft installations have it!! Check your POH before you need to use it, and ask your friendly engineer/mechanic to show it to you next time the cowls are off.
Edit:
Another CAUTION: As the OP suggested, this won't "blow the engine", but may still constitute a level-bust on the Torque/Time limitation graph, as well as threatening your N1 and ITT Red Lines. Again, consult your POH and talk to your engineer/mechanic.
Last edited by DuncanDoenitz; 27th Nov 2020 at 19:39.