When the brother throws a dart on a real board at their base, a "dart fish" materializes only at the moment of impact on Killua's corresponding body part.
Killua is systematically torn apart, suffering massive blood loss as he struggles to comprehend the rules of this invisible game. Strategy Over Raw Power
Killua’s victory is one of the most celebrated strategic moments in the series. Relying on his childhood training—where he mastered darts by the age of six—he deduces the exact rules of the "501" game being played.
Using his Godspeed (Kanmuru) lightning reflexes, he catches the final dart at the millisecond it materializes.
Despite his win, the victory is bittersweet. The blood loss is so extreme that Killua collapses, nearing death in the dark caves. His final thoughts drift toward Gon, apologizing for not being able to continue.