They are not air brakes, as they cannot (or rather should not) be deployed in the air. They are Ground Spoilers. In the cockpit, there is a Flight/Taxi switch which operates them. When lining up for departure, this switch is placed in the Flight position. This causes the Ground Spoilers to deploy. As the Power Levers are advanced beyond 15.5 degrees Power Lever Angle, logic lowers them. On landing, with the Power Levers below 15.5 degrees PLA, and nosewheel on the ground, the conditions are met for them to be deployed.
This system is found on the DHC8's 100 and 400. Beats me as to why they are not on the 300's. The difference on the 400's however, is the logic depends on the main landing gear, not the nose. I stand corrected however as I only know about the 100/300's.