Badminton

The power of a smash comes not just from your arms, but from the coordination of your entire body. It's good to build a foundation with dumbbells, but you need to train your core and be mindful of your leg drive. Daily training is what paves the way.

Spin?

I'm not saying strength training is completely irrelevant, but form and hitting the shuttlecock in the right spot are more important. Are you hitting the smash with just your arms? Smash is hit with the muscles from your waist up. Just training your arms is pointless. Rather, train your core muscles so you can hit consistently from any posture, and while doing that, do bodyweight training with dumbbells or weights attached as you extend your arms. Also, don't forget stretching. If you want to hit fast right away, heavier dumbbells are fine, but for the long run, badminton requires flexible muscles, so training with about 1kg dumbbells is best.