Means when you don't know what your enemy is up to, you first must launch a direct but weak attack, to observe the reactions of your enemy and that will reveals his strategy and that leads to victory.
Song Dynasty China
One day, in the county of Jian-zhou, there was a man who lost a precious object. The local magistrate Chen Shu-ku, was called in to investigate. He questioned several people, but no one could tell him who the thief was. So Magistrate Chen laid a trap for those he suspected. "I know of a temple," he told them, "whose bell has great spiritual power that can tell a thief from an honest man. Since my investigation is at a standstill we must employ the supernatural powers of the bell to solve the matter." The magistrate had the bell brought to the courthouse and displayed in the rear chamber. Then he had the suspects brought in to testify to their guilt or innocence. He explained to them that if an innocent man touched the bell it would remain silent, but, if a guilty man touched the bell it would ring out. After lighting incense and chanting prayers, the magistrate had curtains erected around the bell. Previously he had instructed one of his assistants to secretly smear ink on the bell after the curtains were closed. Each suspect was then told to place his hand through the curtain and touch the bell. As they withdrew their hands Chen would examine them. Everyone's hands were stained except those of one man, who confessed to the theft. He did not touch the bell for fear it would ring.
Means that you will undermine your enemy's ability by causing discord between your enemy's friends, allies, family, soldiers, commanders, advisors and population. While your enemy is too busy with making up the conflicts, his ability to defend or to attack will greatly decrease. Then you will attack and that leads to victory.
Six Dynasties Period China
In the later Wei dynasty, the emperor Tai Wu led a hundred thousand troops against the Song general Zang Zhi. The emperor, with his superior forces, chased general Zhang to the city of Yu where he was cornered. The city was strongly fortified and so the emperor planned to surround it and starve the defenders into surrender. Feeling confident of his position, the emperor sent a cart carrying a large jar of wine to the city gate with the message asking for the traditional exchange of wine before commencing his siege of the city. Zang Zhi knew that he needed to fight a decisive battle quickly, or suffer defeat, saw this as an opportunity. He sent some soldiers to gratefully accept the jar of wine and in turn deliver another large jar sealed with the wine maker's stamp. When a cup of this wine was poured for the emperor it was discovered to be urine. This caused the emperor a humiliating loss of face before his own troops and in a rage ordered an immediate assault on the city. The defenders were well prepared and the bodies of the slain imperial troops piled up nearly to the top of the ramparts.In his haste and anger the emperor lost half his forces.