In two situations you might want to tighten the topping lift such that the weight of the boom is supported by the topping lift rather than the sail itself. First, as previously mentioned, when you are about to lower the mainsail, you can tighten the topping lift to hold the boom up higher out of the way.
The second reason to tighten the topping lift is to prepare for reefing the mainsail. Reefing is the process of lowering the mainsail partway, to a reefing point, to use less sail area when the wind is blowing harder. Tightening the topping lift provides more slack in the sail itself, making it easier to lower the sail partway and secure the reef.
After raising or reefing the sail, however, it is necessary then to loosen the topping lift so that the weight of the boom pulls the sail tight. In the photo shown here, the topping lift is still too tight, causing bagginess in the bottom of the mainsail. This makes the sail very inefficient for sailing.
Go to the next page to compare the sail shape with the topping lift eased.