just to better explain your concept of "sideways moving". Actually, when a board is going, there is always a component of sideways movement. If that wasn't there, the fin simply would not work, or just act as a dragging device. That "sideways component", which in fact explains angle of attack, is higher when pushing hard against the fin and lower when going deep downwind and/or sheeting out the sail. Rails (at least in the tail) are actually almost continuously engaged.