Simple
Mike is right: the toppiing lift is normally slack when sailing. If not it can prevent correct sail trim when close reaching and the main is sheeted in tight, or whenever a flat, de-powered sail shape is desired. Adjust it slightly slack when the main is sheeted amidships, then forget about it.Paul is right: leave it attached and forget about it. Let the racers detach their lifts, shave their legs, and urinate before the race for that imagined extra speed.Joe is right: in very light air, the lift can be tightened a bit to overcome the weight of the boom and allow the desired curvature in the main to create lift. It sometimes makes the difference between ghosting along nicely and resorting to the engine. This is technically a "cheat," since the topping lift is not a sail control, per se., but it is respected as a clever solution employed by a shrewd skipper.