Two extra fins that can be placed on the rail help track which gives extra grip in extreme turns. If you rail the board up hard, the tail starts to come out of the water, for example during top turns (which then turn into lip slides) and in extreme bottom turns. In these extreme situations, the center or twin fins may come out of the water, but one of the side fins will still grab! These side fins have a balanced contour and a straight running curve. They feel more direct and allow more radical turns. The side fins and covers are made for Mini Tuttle Boxes (MT).