It also took me some time to figure out. After the calibration step, the world select screen plays like the normal game: press any key when the red or purple disc is above a tile/square. It will move you to that square. Then it’s a matter of timing and rhythm.
I think the animated GIF on the Steam page (https://store.steampowered.com/app/977950/A_Dance_of_Fire_and_Ice/) show it better than on this page.
I also think the page generally speaking should just explain how it works.