In 6-1, you'll find a fast-moving platform that is impossible to jump on normally. The puzzle piece is on this platform. To reach it, you need to drop the time-slowing ring near the platform's path. As the platform enters the bubble, its speed will drastically decrease, allowing you to easily jump onto it. Once on the platform, you can pick up the puzzle piece. Remember that the ring's effect is localized, so you need to place it strategically to affect the desired object. You can also pick up the ring and re-throw it if your initial placement isn't ideal.
