A fast pull trips the retractor
Inside the belt’s retractor, a spring keeps gentle tension on the webbing and winds slack back onto a spool. As you lean forward slowly, the spool turns freely, letting out just enough belt for the movement.
A sudden lunge makes the spool spin much faster. In many retractors, that rapid rotation moves a small weighted part outward; it pushes a locking pawl—a catch—into teeth on the spool or its frame. The catch stops the spool from turning, so the belt can no longer feed out. It is the speed of the pull, not simply the fact that you leaned, that triggers this kind of lock.
The car can trigger it, too
Many seatbelts have another way to lock: a sensor responds when the vehicle slows or tilts sharply. In common designs, a small pendulum or weighted sensor shifts during hard braking or a collision and trips the same general kind of catch. That lets the belt respond even if the webbing itself was not pulled out quickly beforehand.
These mechanisms are designed to allow ordinary reaching and posture changes while restraining the belt when motion becomes abrupt. The exact design varies, but the goal is the same: keep the retractor from paying out more webbing when the occupant is being thrown forward.
That is why a quick tug while the car is parked can make the belt seem stuck. Ease the pull, let the mechanism reset, and then draw the belt out smoothly.
