[bede]

The seatbelt is an excellent example since there are multiple mechanisms with varying observability contributing to its effectiveness. Even after discovering the dynamic locking behaviour, a naive yet inquisitive passenger remains blind to the pretensioning feature that is observable only during a collision.

[gregmac]

Another is why they lock when you pull them out all the way: I discovered this accidentally as a kid, but just learned the lesson "don't do that" and didn't really think about it further.

It wasn't until I had to install a child seat for the first time when suddenly it became clear this was a very useful feature.

[bentcorner]

It's an ingenious design. Given a seat belt that cannot be intentionally locked in this way, how would you address complaints from users who want to arbitrarily lock their seat belts? To me, "pull it out all the way" isn't an obvious choice, but is such an elegant solution.

[borski]

Thank you! I have wondered this myself and mostly just fine with “it’s annoying so I make sure not to ratchet it anymore,” but since I don’t have kids (yet?) — thank you for the explanation!