In Kirby Tilt 'n' Tumble, there is a position option where I can select "Flat" or "GB". What does this position option do?
This is an option to define in which position the gyro should get considered centered, i.e. Kirby is not moving.
For gyro controls, the original cartridge contained a sensor able to detect rotation (the big advantage cartridges had over other media).
However, there were two typical use cases in how to hold the Gameboy:
- Hold the Gameboy upright, with the game cartridge facing away from the player. ("GB")
- Hold the Gameboy flat, with the game cartridge facing the ground. ("flat")
The two in-game options represent these two styles, so you're able to pick the one matching your way of holding the console to use the gyro properly.
This isn't too different on how many people hold their phone when interacting with the screen. I think they even made the Switch version follow this very same option, too, although I don't think I've ever changed it while trying the game.