I bought these on the weekend and used them for the first time this morning. I have a partial tear in my shoulder and can't do the front crawl (yet), so I've modified my swim to keep my arms under water. (think dog paddle but with arms only slightly bent at the elbow) The package says you'll know if your stroke is correct after one length of the pool because the paddle will shift and/or come off. I had no trouble keeping these in place while swimming. I wasn't sure how they'd work for me because I wanted more resistance but not so much that it would further injure my shoulder. I absolutely love these! They're small, compact, curved just right and I will be using them from now on!