Originally Posted by SkiDoc

The rail is what your skater is resting on. All cats will have these. They are what allows catamarans to carve turns. Imagine making a left turn. As you turn the left sponson will dip somewhat the boat will want to slide to the right. As the boat starts to slide surface water is forced up the inside face of the sponson til upward flow is blocked by the rail. This allows the left sponson to carve or hold its inside position. At the same time the right sponson is sliding along the deadrise of its sponson. Hooks or spin flips occur when the rail is lost.