If you have a passenger in the right front seat the car does:
1. Checks for occupancy. Does this through a weight sensor in the seat. IF there is someone there it:
2. Checks for seatbelt engagement. If the seatbelt is buckled, then the threshold for bag deployment is raised. Side bag sensor in the door is activated. Explosive charges in the seatbelt retractors are also activated and ready for firing.
If there is no one in the passenger seat, then the front passenger and side bag are disabled to save the bags from deploying when not required. The seatbelt retractors are also disabled.
Your cars uses a multi-plane accelerometer (sp?) to check for sudden impacts. If one is detected, then the occupied seats have their bags deploy depending on impact. There are also crash sensors located in the bumper (rear I'm not sure?) and doors. It uses all this information to decide whether or not to deploy the bag or belt tentioners, and which ones.
Newer cars can also fire the bags at different speeds to compensate for different g-loads during impacts.
Also, in BabySmart equipped cars, the seat has a radio transmitter that talks to a BabySmart child seat. When a childseat is detected, it disables the airbags and belt retractors. The best thing is NOT to put them in the front seat...
1998 C230 "Black Betty"