So the power levers can not be moved below the flight idle gate in flight, without lifting the triggers and the siren sounding?
The power levers cannot be moved below flight idle at any time on air or ground without lifting the triggers.
If the beta warning system or beta lockout system is fitted a siren will sound when either trigger is lifted with weight off wheels and above 20ft radalt
Are the "triggers" and the "beta lockout mechanism" one and the same thing?
No.
"beta lockout mechanism" is not a great description. Comes from the AD requiring a "mechanism: , ie a way to, to prevent operation in beta in flight.
It is actually an electrical circuit which will activate to coarsen the blade pitch when low blade angle ( = high rpm) is detected. It works by actuating the feather solenoid to cause the prop to move toward feather, when blade angle has coarsened the required amount the solenoid deactuates and the blade will move fine again triggering the beta function again so it will move coarse/fine/coarse/fine in a small range. The ground range lights for the respective engine will be cycling at this stage on the pilots glareshield.
I'm not that familiar with beta lockout as of the 40 odd dash 8s i've worked on over the years, i've only seen one with it fitted, like i said, not mandatory in Australasia - although maybe not for much longer.
BTW - all this above is how the system works on a serviceable aircraft.
I'm not postulating any whys with relation to the accident, I don't even know which option was fitted, if any.