As the line is worn, or breaks off, a bump feed string trimmer the operator knocks the reel on the ground so a release mechanism allows some of the line in the reel to replace the spent portion. Newer models "auto-feed", where a small cutter ensures the exposed length does not exceed what can be swung efficiently. Newly extended line operates more efficiently because of its heavier weight and surface effects. The speed of the spinning hub is controlled by a trigger activated throttle on the handle.