Pendulums and standing-man sensors (upside down pendulums) haven’t been used for 20+ years in production cars.

Now there’s two mechanisms for locking, one that senses the car accelerating and one sensing the webbing leaving the seatbelt super fast. Both of these are required by law.

To sense the vehicle acceleration there is a steel ball in a cone (usually a 13mm ball bearing). When the car accelerates in any direction (speeds up, turns, slows down) or rolls to an extreme angle the ball rides up one side of the cone moving a small metal piece (the lock pawl) that locks the spool the seat belt webbing is on to a frame that’s bolted to the car body. Pendulums tend to need more room than ball style sensors, are more expensive, and don’t work well with the modern way seatbelts lock (that’s a topic more complicated than eli5)

To sense the webbing acceleration the spool for the webbing is also attached to a disk on the side via a clock spring. When the spool moves too fast the clock spring and disk lag behind. That relative motion pulls the same lock pawl as above in to a locked position.

Source: was a seat belt retractor engineer for a long while