As pictured, I have it on a 1988 racket. It actually enlarges the sweet spot.

An interesting product. I've been using this for several months during my tennis lessons and I am definitely more consistent in hitting the sweet spot. It is frustrating at first because if you miss that coveted sweet spot all the power is sucked out of the hit... However that is also the point! There is a big incentive to hitting the sweet spot. I am also able to generate more power after removing the training aid because of the drag the aid creates (or lack thereof with the aid removed).Be patient with yourself in the beginning and remember to focus on the racket making contact with the tennis ball. I would recommend using it during the warmup drills and take it off before getting into the main lesson drills. This suggestion comes primarily because the drag the aid creates will change your swing timing ever so slightly and if you practice with it through the main drills it is possible (as in my experience) that your shots that were perfect with the training aid on will be a little off after taking it off. Do give yourself some time to adjust after removing the aid. For the amateur 10-15 minutes of using the aid per lesson will significantly improve your swing power and increase the percentage of hits in the sweet spot.It is a training aid so remember what it is designed to do and how it accomplishes it. With this in mind this aid can be used very effectively.