What mechanism do safety belts use to engage?

Earlier today I came across a creep testing machine that uses a safety belt as a mechanism for stopping loads from hitting the ground. It is probably the first time I saw a safety belt roll outside of a car and it got me wondering how do safety belts work?

What is the primary mechanism that they are using for engaging and disengaging? I expect it to be entirely mechanical in nature, however I can’t put my finger on it.

Is it at all similar to the ratchet or the freewheel mechanism?