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I don't know if anyone knows, I can only guess as written: Two audio sources are panned left and right. How wide is the stereo image?
They implement two audio sources from the same instrument, each a bit different, and pan them to each's own channel. That is fact.
1 ) Afterwards, my guess is that they probably use a plugin / algorithm to change the mid/side correlation by messing with the phase at a certain amount (as you do with mid/side config).
You declare that amount with the game UI. The click of the mouse sends your answer from 0 to 1 and checks if its in the boundaries of the actual answer.
2 ) They could also do something different and simpler, and actually pan 2 almost similar mono sources by an amount and the same idea goes with you choosing the answer, and the game checks if its in the boundaries.

I am not very knowledgable in the topic, you could send them a support ticket if you really want to know (If they'll even tell you, but you can try). Cheers.

Good job @Maciek Sliz 🏆

Way to go, Maciek! Congratulations for winning your Golden Ears!

Fantastic work!

Thank you all!

Congratulations!

Outstanding accomplishment!

Very nice! ⭐⭐⭐

Fantastic!

Thanks a lot Cuantas Vacas! Gracias!

Congratulations!

Congrats man! Gj!

Awesome work!

Jacques, that's an excellent accomplishment - congrats for winning your Golden Ears!