I STILL have no idea how or when to use my topping lift OR my rigid vang:confused
One of the most misunderstood aspects of sail trim is "twist." The wind at the top of the mast is faster than the wind at water level, due primarily to drag. The top of the sail needs to be out more than the bottom to properly trim for this. As wind speed increases, there is less speed differential, and less twist is needed.
On boats with proper travelers, while close reaching and close hauled, leech tension and twist is controlled by the mainsheet, and angle of attack is controlled by the traveller. However, in light wind, it is common for the weight of the boom to weigh down the sail, over-tensioning the leech, and reducing twist. In higher winds, the pressure of the wind on the sail is adequate to overcome the weight of the boom. Also, in higher winds, less twist is needed anyway, so more leech tension is required anyway.
So, a proper rigid vang, or perhaps a soft vang combined with a Boomkicker, can lift up the boom. In the absence of a rigid vang or Boomkicker, an easily adjusted topping lift can also lift the boom, and allow the main to twist off.
If a topping lift is not easily adjusted on any point of sail, or if it's tied to the boom, then all it does is hold the boom up when dousing sail, and it will not aid sail trim. It also probably hooks on the leech during tacks.