I saw the hook thing and thought. Wow a multi-tool with a chain tool. That hook thing is used to hook one link of the chain on each side, you do this to create a loop below the hook think that is not under tension. This allows for you to break the chain in this spot and not constantly be fighting trying to keep the chain loose while breaking or when trying to assemble the links again. At the home shop a wire coat hanger can be cut to do the same thing. It has a prized place in my toolbox!