system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why and how does the seatbelt lock at certain points and not others?

I feel like I can't be the *only* person who has had the misfortune of attempting to bend forward slightly to retrieve something off of the floor just to be choked by a locked seatbelt. So, why does the seat belt lock then, but if you release tension and then lean forward again, it doesn't lock? What determines this pattern?
gpt
The speed at which you pull the belt. In its very simplest form the seatbelt is rolled up somewhere and that roll has some teeth that go outwards if it turns too fast and grab into locks/notches in the surrounding metal. If you pull normally, everything goes smooth, if you pull fast (like in a crash) it locks. Once you release the tension, the teeth snap back in and you can use it normally again.