It's a double loop string, and you screw the tuner all the way down, put the new string on, and then screw it up again to tighten, and tune. At the top there is a row of knurled nuts on the end of a threaded shaft. On this shaft there is a collar with the hook for the string attached to it. The hook goes through a slot in the front, and this keeps the hook from rotating when the nut is turned, and guides it up or down the threaded shaft as required. It's a fairly primitive, but effective system