Originally Posted by sailingdog
If you don't leave the topping lift a bit slack when you're sailing, the mainsail won't ever take proper shape, since the leech will be baggy. The boom's weight normally flattens out the leech of the mainsail—with the topping lift tight, it doesn't flatten out properly.
Also, a boom vang won't work properly, to flatten the mainsail in heavier winds, if the topping lift isn't slack.
I didn't know that. I assumed the reason for slacking it was to prevent chafing or obstruction of the leech when tacking or jibing.