Before the days of winches, to get the last bit of hoist, particularly with halyards, to avoid any slackness when setting of a sail by hand. A halyard was sweated up by taking a single turn round a cleat, hauling the standing part out from the mast horizontally while keeping tension on the end to prevent it slipping on the cleat. This raised the sail fractionally. The slack was then taken up round the cleat as the halyard was released so that the extra hoist gained was not lost.
Subjects: Maritime History.