Why does Katniss think the poisonous berries will end the games with District 12 tributes as winners?

Why does Katniss think the poisonous berries will end the games with District 12 tributes as winners? She says to hold the berries out because she wants everyone to see. She wants to make sure that the Gamemakers understand what she and Peeta are about to do and that if the Gamemakers allow this to happen, the Hunger Games will not have a victor this year.

She says to hold the berries out because she wants everyone to see. She wants to make sure that the Gamemakers understand what she and Peeta are about to do and that if the Gamemakers allow this to happen, the Hunger Games will not have a victor this year.