I saw the Flat Ball trainer at a pro shop the other day and thought they were a good idea. Essentially, the soft, flat disks teach you to pick the ball off the ground. If your swing comes in behind the ball, your club will skip up over the disk. If you come in on top of the ball, you’ll flip the thing into the air. They only way you can get the disk to fly forward is to hit it on the quarter inch wide high side. They’re soft, so you can theoretically use them indoors.