A salmon fly tied around a tube, which is slipped onto the leader before the hook is tied on, the tube being left to run free on the line above the treble hook. The advantage of this is that the tube fly is pulled down against the hook by water pressure while being fished, but when the hookis taken and the line pulled in the oposite direction, the tube has the ability to ride up the line and out of the way of the fish - this is useful when unhooking.
The tube is usually of metal - brass or aluminium - but a plastic lining saves the friction on the line and is better for shallow water summer fishing.