This happens with every control that was manual in the real aircraft.
So, if for example you want to extend the gear by using an emergency hand-pump mechanism you'll have to hold down the appropriate key as well.
This is also how the bomb bay doors work in the Ju88, because in the real one the bombardier had a hand-crank to manually open them. In other bombers that use an electric/hydraulic/whatever motor you just press the key once.
Hope it helps
