All shoulder belts typically have an emergency locking retractor. With this type of retractor, during normal driving you can lean forward and back and the seat belt will slide in and out, but when you slam on the brakes in an emergency, the shoulder belt locks and holds you tight.

Yes, modern seatbelts are designed to lock during heavy braking through two distinct mechanisms: Webbing-sensitive and Vehicle-sensitive locking. The webbing-sensitive mechanism (the "jerk" lock) uses a centrifugal clutch that engages if the belt is pulled out too quickly. The more relevant mechanism during braking is the vehicle-sensitive one, which features a weighted pendulum or a metal ball in a cup. When the car decelerates rapidly (brakes hard), inertia causes the pendulum to swing forward or the ball to roll, which triggers a locking bar to jam into the retractor's gear, instantly stopping the belt from loosening. In 2026, many high-end vehicles also feature "active" pretensioners that use sensors to detect an impending collision and electronically tighten the belt even before the physical braking force fully kicks in, ensuring the passenger is safely pinned against the seat back before an impact occurs.