To understand why **airbags** and **headrests** can lose effectiveness if the seat belt is not fastened, it is important to know how these passive safety systems work.

- **Seat belt**: Its main function is to protect vehicle occupants in the event of an impact or rollover. It keeps people in their seats and prevents them from being ejected, which significantly increases the chances of surviving an accident. To be effective, it must be properly adjusted and attached to the vehicle's body.

- **Airbag**: This device activates in the event of a collision to cushion the impact and protect vehicle occupants. However, if the seat belt is not worn, the body can move uncontrollably, reducing the airbag's effectiveness. The seat belt helps keep the occupant in the correct position so that the airbag can deploy properly and offer maximum protection.

- **Headrest**: Its function is to prevent neck injuries, especially in rear-end collisions. If the seat belt is not fastened, the body can slide forward or downward, decreasing the protection the headrest can offer.

In summary, the seat belt is crucial for both the airbags and headrests to function optimally and adequately protect vehicle occupants.