How do the frontal airbags know when to inflate?

The frontal airbags are designed to inflate in moderate to severe frontal or near-frontal crashes when the impact exceeds a predetermined deployment threshold. If the vehicle goes straight into a non-deforming wall, the threshold for reduced deployment is approximately 12–16 mph (19–26 km/h), and full deployment is approximately 17–22 mph (27–35 km/h). Deployment depends on what the vehicle hits, the direction of impact, and how quickly the vehicle slows down — not simply on speed. (Page 76–77)