It is most likely triggered by an accelerometer chip, as are airbags. Once the chip sensors are tripped by a sudden shift of movement/direction that falls within the parameters of an accidental impact, it in turn trips a solenoid that instantly releases compressed gas into the bag. With the eyes left uncovered by the air bag, even a malfunction would be harmless, though mighty startling. A very clever redesign of existing tech, and would encourage more riders to always have a head safety device on while cycling.