>>536361005
It's explained elsewhere but what Seto made Gozaburo agree to was a 2v1 Chess game between him, Seto and Mokuba.

Now this is obviously a stupid idea because all you need to do to win is simply repeat the moves the opponent does right back at them so they're playing against themselves and this is exactly what Seto exploited which makes no sense since Gozaboru as a chess grandmaster should have been aware of this obvious exploit of the game but I guess for the sake of the plot he let it happen.

You anons can try this out yourselves on chess.com with 2 open browser windows. Start a different game against each on each browser window and be black on one window but white on another window. All you need to do is copy the moves one of the other AI does to win.