Vang and Topping lift
With an adjustable rigid vang, you can get rid of the topping lift altogether. The rigid vang should be able to support your boom when not sailing, the job normally done by the topping lift. This is not, however, the sole purpose of the vang. The Vang allows you to adjust the leech tension, and twist, on your mainsail once the boom goes outside the limits of your traveler, such as on a reach or run.The spring loading in the vang that supports the boom also allows the boom to lift when you need to open the sail in light air, up wind.