That, Is a fishing weight. It is used for bottom fishing. The reason for that design is, When a fish takes your bait he will "run" a bit. A lot of fish are keen and if feel any type of resistence they will "drop" or spitout the bait. When a fish "runs" with the setup the weight is designed for the fish rarely feels that resistence. The line just runs through the hole on the lead where the weight stays put on the bottom until you rip his lips! lol