A piece of hardware with a spring loaded piston that closes when released. Once common on trim chains, however, since it is a piece of cast and unrated hardware, it should be removed from any application in overhead rigging and replaced with appropriately rated hardware.
http://www.neonhusky.com/JShop/product.php/249/0
http://www.neonhusky.com/JShop/product.php/249/0