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The book tells us that this particular pillar is produced in response to one from Barad-dûr. The exchange is essentially Sauron (at Barad-dûr) asking "Are you ready?" and the Witch-King at Minas Morgul replying "Yes!"
Aqui está a passagem relevante de " As Duas Torres " que parece confirmar que isso foi um "Estamos prontos" ou "Nós reconhecemos a ordem de sair":
The great rumbling noise, louder than ever before, rolled in the ground and echoed in the mountains. Then with searing suddenness there came a great red flash. Far beyond the eastern mountains it leapt into the sky and splashed the lowering clouds with crimson. In that valley of shadow and cold deathly light it seemed unbearably violent and fierce.
...
And Minas Morgul answered. There was a flare of livid lightnings: forks of blue flame springing up from the tower and from the encircling hills into the sullen clouds.
...
Across the narrow valley, now almost on a level with his eyes, the walls of the evil city stood, and its cavernous gate, shaped like an open mouth with gleaming teeth, was gaping wide. And out of the gate an army came.
Como uma nota lateral, para aqueles que não perceberam este detalhe específico entre a parede de citações - enquanto a cena do filme estava no ROTK (conforme a pergunta), a cena equivalente está no livro "The Two Towers" .