Sherry L. Werner, who has studied fastpitch players for over 20 years, did a study on the Finch windmill a few years back. Her assessment was that after testing college players during a 6 week study, that it did indeed build up speed as advertised. College pitchers, who should have full power already, actually gained between 3-5 mph within that time span when using the Finch as per directions. It went on to state that it will NOT help mechanics as it does not follow good form when using it. It works on ALL muscle groups of the shoulder and arm that is used for pitching.
A friend's daughter used it and gained speed while she was following the plan. Once she stopped using it she gradually lost the speed that she had gained during that workout time.
Hope this helps you. You can look up Sherry L. Werner on the net and I think it will take you to that study.
Good luck P.S. They are expensive devices.