that's the self tacker. The jib sheet is attached to a car on that curved traveler track so that when you tack back and forth, the jib automatically slides over to the correct side and you don't have to touch anything. It's become the standard for most new cat designs. The only drawback (well...that's debatable) is that you cannot have a jib that overlaps your mast and mainsail because it has to stop short of the traveler track to work properly.