Ok,
So here it is in a sort of timeline that I take it you're after.
When you crash into a wall (or another car) at 50~60 km/h, several things happen:
1) Car detects massive deceleration through sensors in bumper/ front end.
2) Car computer recieves the deceleration information from the sensors and processes it, sending commands to the safety systems throughout the car.
3)Seatbelt pretensioner's fire, pulling the seatbelt's tight around your body, helping it to stay 'with' the seat and give it a more controled deceleration.
4) The the airbags inflate, as the airbags inflate, the active headrests move forward (and upwards in some cars) so they stay closer to your head to control the rearward motion at the end of the crash.
5) If the forces of the crash through the seatbelts exceed the safe limits that the human body can withstand, the force limiters built into the seatbelt base, (either where the seatbelts attach to the chassis OR where they attach to the pretensioner/ inertia reel mounts) will "release" reducing the force applied to your body by the seatbelts. Most of the time, the force limiters are built to work in the same action that the lid peels off a sardine can.
This is a pretty basic description of how it works so I apoligize if it doesn't seem exactly correct, but if you have any other questions about what I've tried to explain, ask me and I'll explain it in more detail