Understanding the Arrow's Power and Doppio's Duality in JoJo's Bizarre Adventure Part 5

Understanding the Arrow’s Power and Doppio’s Duality in JoJo’s Bizarre Adventure Part 5

Reader’s Question:
I’m currently watching Part 5 of JoJo’s Bizarre Adventure. I’m at the part where Diavolo and Polnareff are fighting in the Colosseum. What kind of effects does the arrow that Polnareff has have? I’ve been paying attention, but I still don’t quite understand that part… Could you please explain