The effect of water pressure pushed against a surfboard's surface, which creates acceleration down the line on a wave. This is the simple way of describing drive and its immediate effect. Looking at it more closely, we see that "drive" in a surfboard context implies a couple of factors. - First, it's about pressure. Specifically, water pressure working against a surface. To harness the pressure, you've gotta have a surface for it to work against (ie. , a fin). - Second, it's about direction. Drive is aimed; it's purposeful, not random. Drive doesn't have an opposite so much as a corollary, which is Drag. Drag results from friction between waterflow and wetted surface, and it's not altogether a bad thing; without some elements of Drag, as without Drive, a surfboard would be virtually impossible to control. (Best example I can think of: a surfboard without any fins at all. ) Almost without fail, wherever you create the possibility of Drive, you'll also have the possibility of Drag. Getting that balance right is the key to great surfboard design. A middle fin adds Drive and Drag at a central point of a surfboard's tail. This adds control and direction, providing an anchor for turns. In the classic Thruster setup, the side fins are reduced in volume in order to balance the design. Take the middle fin away, and both Drive and Drag are removed; waterflow gets past the fins more easily, giving the board a skatier, skimmier feel, but some control and direction is lost. This is only partially made up for by the larger fin size of the classic Twin-fin design.