There are systems triggered by the car's movement and there are systems triggered by the belt's movement.
The first sort of system locks the spool when the car rapidly decelerates (when it hits something, for example).
The central operating element in this mechanism is a weighted pendulum. When the car comes to a sudden stop, the inertia causes the pendulum to swing forward. The pawl on the other end of the pendulum catches hold of a toothed ratchet gear attached to the spool. With the pawl gripping one of its teeth, the gear can't rotate counter-clockwise, and neither can the connected spool. When the webbing loosens again after the crash, the gear rotates clockwise and the pawl disengages.
Like Murci-me said: the pendulum type has to be mounted exactly as it was in the original oem position to operate properly.
The second kind of system locks the spool when something jerks the belt webbing. The activating force in most designs is the speed of the spool rotation.
The central operating element in this design is a centrifugal clutch -- a weighted pivoting lever mounted to the rotating spool. When the spool spins slowly, the lever doesn't pivot at all. A spring keeps it in position. But when something yanks the webbing, spinning the spool more quickly, centrifugal force drives the weighted end of the lever outward.
The extended lever pushes a cam piece mounted to the retractor housing. The cam is connected to a pivoting pawl by a sliding pin. As the cam shifts to the left, the pin moves along a groove in the pawl. This pulls the pawl into the spinning ratchet gear attached to the spool. The pawl locks into the gear's teeth, preventing counter-clockwise rotation.
The "pull lock" type should work in a modified position but MUST BE TESTED IN THE MOUNTED POSITION.
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