That is kind of true. Seat belts are inertia activated inside the retractor. They usually will not lock unless the car is decelerating rapidly, or the car is turning with a good lateral load on them. This activates the mechanism that locks the belts. Pulling on the belt will not lock it. One way to see if they catch is to go say 60, hit your brakes hard (as long as no one is behind you), and see if your body is stopped by the belts. They should lock and keep you from moving forward. Belts are designed this way for comfort to allow you to move around in normal circumstances. If they locked all the time, people would just unbuckle them to move around and end up not wearing them.
Belts from moderately or severly wrecked cars are usually considered not safe since the webbing may have been stressed by the accident and they could fail if involved in another accident. Minor accidents usually do not damage the belts.
The real problem we have with Fox belts is they don't retract worth a darn when you take them off.
