Designs around me - 2

While in a train trip

 A few days back I had a journey on the train and while traveling I noticed this thing used to make uplift of the middle berth. What I got attention is the small U-shaped metal piece used to lock the chain. Very simple and small thing but so powerful and efficient in its duty. 

While the berth is not used, the chain will be hanged (image 1) and the U shape of metal will be facing down allowing the chain can be used by unhooking at any time (that shape of groove helps easiness in unhooking). But while we connect the chain to the berth, the down-hanging metal piece prevents the unhooking of the chain (image 2)

No use of automatic or any complex lock mechanism, just a piece of metal, a special type groove, and the power of gravity...Done! the problem gets solved