In 1137, General Yue Fei of the Southern Song got an order to stage a war against the Jin army. Yue Fei knew that there would be a rough battle now that Liu and his new master formed an alliance. But he also knew that there was a conflict between General Liu and Jinwuzhu, a general of Jin. Yue Fei would take advantage of this and weaken the enemy force by driving a wedge between the two generals. But how?

It so happened that Yue Fei received a report that a spy sent by Jinwuzhu was captured. Hearing this, Yue had an idea. He sent a word that he would see this spy.

When the spy was called in, Yue pretended to know the spy and said to him reproachfully, "What has happened to you, Zhang Bing? A few days ago, I ordered you to take Liu Yu a message expecting to collaborate with him to kill Jinwuzhu, but I haven't got any information from you. Where have you been? I am thinking of sending another man to complete the mission."

The spy felt puzzled, but he soon believed that Yue Fei had mistaken him for someone else. He said to himself, "Why don't I make the best of this mistake by taking this important message to my general?" He suddenly knelt down and said beseechingly, "Pardon me, General. Give me one more chance and I promise to hand the message to General Liu this time."

"Well, I pardon you", replied Yue Fei, "I give you another chance. But if you fail again, you can't escape bad luck." He picked up a writing brush and wrote a letter including an elaborated false scheme to kill Jinwuzhu. To make the wedge plan more credible, Yue wrapped the letter in a wax ball. Then he cut open the skin on the spy's leg and put the ball in the cut.

The enemy spy was so excited to get the information. He eagerly ran to Jinwuzhu and handed the letter to him. Jinwuzhu was greatly shocked when he learned that Liu Yu had been plotting to kill him. He immediately reported the message to the emperor. Very soon, Liu Yu was deposed and Yue Fei succeeded in weakening the enemy force and paved the way for his defeating the enemy.