Some top sailors put two or three of these on their mainsheet bridle. They clip their mainsheet onto the forward ring in light air, so they can pull the mainsheet and have less mast bend. This keeps the sail fuller so it provides more power in light air.
Then they clip their mainsheet onto the back ring in heavy air, so when they pull in the mainsheet they put more bend into the mast. This makes the sail flatter, which means less power to shed through luffing, lowering the sprit pole, etc. This helps you keep your boat sailing flat and fast.