Airbags in cars are triggered by a sensor in the front bumper (or side in some cars) that inflates the bag before the occupants of the car begin rapid deceleration. How would the helmet bag be triggered before the helmet hits the road?

An airbag deployment is an explosion that takes place in a split second within the confinement of the airbag. For this to work on a helmet, it could have a gyroscopic sensor, which can sense if the helmet is projected at an inappropriate angle or speed. It could also have a pre-impact sensor, that could sense whether it is going to hit any solid as well as the rate of impact.

how about a helmet that's always inflated. like a beachball.