The front cannon is the carriage
重炮 stacks two cannons on one file to the king: the front cannon already sits at the palace heart e8, and the rear cannon — arriving on the same file — fires THROUGH it at the king. Count the escapes and each one fails: the king cannot capture on e8, because with the front cannon gone the bare file gives the rear cannon a clean shot, and the soldier on d8 guards the square besides; the advisors can neither interpose on the file nor capture the front cannon without becoming the new screen themselves. The rear cannon is still out on the wing: stack them. The proverb says it all — 重炮无垫子: against stacked cannons no interposition exists.