If the ACU detects a collision through these sensors, it “tells” the initiator to ignite the solid propellant or chemical explosive
For example, most airbags use sodium azide (NaN3), potassium nitrate (KNO3), and silicon dioxide.
When ignited, the sodium azide decomposes to produce nitrogen gas and sodium metal, which then reacts with the potassium nitrate to release more nitrogen.
Both reactions are exothermic, producing large mounts of heat. The hot nitrogen then inflates the airbag. The entire process takes only around one-twenty-fifth of a second.