The discussion needs to start with how mast bend and forestay sag control the power of the main and jib, respectively. As a quick review, there’s inherent shape built into sails using what’s called “broad seam,” whereby the panels that make up the sections of the sail are designed and cut with curves on the connecting edges, or seams. There’s not much we can do to change it on the fly. The rest of the shape is built into the main and jib with luff curve and luff hollow, respectively. For the mainsail, luff curve is extra cloth added into the luff, in an arch from tack to head. When the mast is straightened, it pushes that material into the sail, adding depth. When the mast is bent, it pulls the extra cloth, and thus shape, out of the sail. The same is true with the jib, except cloth is cut out of the luff in an arch from tack to head to create luff hollow. When the forestay is sagged beyond that luff hollow, cloth — and shape — is pushed into the sail. When it’s tightened, cloth is pulled out of the jib luff, thereby flattening the sail.