Thumb strengthening while using Golflexx creates faster texting! (kidding).
Our specific grip instruction and thumb / finger placement is meant to keep users from over-flexing their Golflexx. During the R&D process we handed Golflexx units out to between 15 and 20 PGA professionals at a local tournament here in Oregon, the Jeld Wen Tournament, where Golflexx sponsored a booth. During that time we took advice from many pro's who tried Golflexx out on the range. It was an awesome day as we watched some very big names using our product. It was suggested that thumb placement should be specifically put on the grip as such allowing the user to get maximum benefit from the pre-loading process. Our spokes person Dr. Donald Fox agreed.
You can, however, hold the ends with your thumbs wrapped around the grips once you get used to your flex limit. There are several recommended exercises that require your thumbs to be on the grips as per the instructional DVD but adjust them according to your comfort level, just don't flex more then the apprx. 6 degree limit. Keep in mind pre-loading your unit is meant for stretching and not so much for strength training although you will get some benefit to strengthening the muscles used.
Hope that helps and thank you all for your input and support.