There is a ratcheting mechanism in the seat belt that is supposed to lock up when the belt is pulled out too fast. If the mechanism is locking up when the belt is pulled out slowly, as in the passenger leaning forward, then the seat belt assembly may need to be replaced.
One thing to be aware of: Volvo has seat belts that are supposed to lock up, but I believe they are only in the rear. These are for locking in baby seats. You are supposed to pull the belt out all the way, then when you retract the belt, you will hear a ratcheting sound, and that means you have activated the locking mechanism for the baby seat installation.