This 'Hook + Pin' fin was designed for fin security in mind. Too often, poorly constructed inflatable paddle board fins will fall or scrape off and be lost in the water. Our specially designed hook is created to secure the fin in place with more strength, and comes with a securing pin attached to the fin with a string. You hook the fin into your board fin box, pull the fin back and then secure the fin to the board with the pin. This fin is compatible with all Tower inflatable paddle boards. This is the same fin that is sent in all original Tower iSUP packages.