This is a safety and securing device used in cable pulling applications. The figure-eight design opens sideways and once connected to the pulling equipment, it won't open because it snaps shut. NOTE: The sister hook is made from malleable iron and will be the weakest link if used with steel devices. They are sold in sizes by the eye opening and each size has a different safe pulling load before it will shatter. They are available in 9/16" (430lbs) through 1 1/16" (2065lbs).