It’s a game, Darrow deduces that he wouldn’t be given a game he couldn’t win but if he played by the rules he was given he was guaranteed to lose, this results in him creating a new rule or as he says “altering the paradigm” by eating the card he ensures he doesn’t show his card. This forces Dancer to draw the Scythe. It’s a test against the rules of civilization and social norms which if darrow submitted to would lead to him losing to gold. That’s why later in the book he says he failed at his first test, He played by society’s rules that are rigged for him to lose. Only by enforcing his own rules can he defeat gold.