With the mainsail fully raised or reefed, the topping lift should be just loose enough so that the boom pulls the sail taut. As shown in this photo, the topping lift now is looser and hangs limply on the near side of the sail’s luff (the trailing edge). The boom pulls down on the sail rather than against the topping lift. This allows the mainsail to achieve a good shape and to be trimmed well for sailing at different points of sail.
The topping lift should not be so loose that it flops around and may get snagged on sail battens or other rigging. Being just a little loose offers another advantage: if you forget to tighten it up before lowering the mainsail, the boom will not drop as far down—with less risk of hitting someone’s head!