Anti Drag Insert is designed to fit into a ProBox not being used with a fin. The Anti Drag Insert covers the slot opening of the ProBox with no fin which allows for a smooth water flow over the bottom of the board. The Anti Drag Insert is designed to float, which adds positive weight back into the board and allows the board to feel like NO box is there when your board is in the water. This I find to be great in Multi Task boards with multiple boxes to enjoy endless fin options without worrying about tail weight. Because weight out of water is different than weight in the water," WE" do not surf out of water!