One of the little-known secrets of great swimmers is learning to relax the hand and wrist on the recovery of the backstroke and freestyle. The ability of the swimmer to relax the hand and wrist as the arm swings over the water for the next stroke enables the arm muscles to recover more for the next underwater pull. It's also what makes the fastest swimmers look as if they're not working so hard when swimming so fast!
World champion backstroker Junya Koga demonstrates how this is drill is done with fins, enabling him to relax his entire arm during the recovery phase in this video from TheRaceClub. The video takes an in-depth look at the "Six Kick Switch" swim drill. The drill is explained eloquently and concisely by coach Gary Hall Sr so it's easy to try it for yourself!