◇ Just read — no move neededThe hollow file
A cannon on the enemy general's own file with an empty road between them is the hollow cannon (空头炮) — the most feared cannon posture in the game. It gives no check yet, because a cannon needs a screen. But the file is a loaded trap: ANY piece that steps onto it becomes the screen for a check, so the defenders must either hold the file forever or feed it one body after another. This is exactly what the central cannon opening (中炮) plays for from move one — 炮二平五 aims at the very file you see here, and every early exchange decides whether the hollow file opens.