Dash 8 has triggers under each power lever that must be lifted to move below flight idle gate.
There are 3 options with regard to prevention of beta operation in flight.
The AD above refers to the beta lockout mechanism which prevents the movement below flight idle in flight. This is only mandatory for FAA certified aircraft - not in Australasia
There is also an option of just a beta warning horn which sounds a warning siren if the triggers are lifted.
Third option is none of the above.