- on the "thumb" part of the handle, there should be two interlocking pieces of cogged belt:
        http://www.nationalhose.com/item_images/cogged_belt.jpg

    Before wind-up, put the end of the string here (possibly with some figure-eight knots), and
    press down firmly on it with your thumb.

    To release, release with your thumb.  No spring is needed, because the tension on the line
    should pull the two pieces of cogged belt apart.




- THUMB -- or it could be made of the "rack" of a rack-and-pinion, two of them, face-to-face
        http://www.mcmaster.com/#racks-and-pinions/