When the enemy is too strong to be attacked directly, then attack something he holds dear. Know that he cannot be superior in all things. Somewhere there is a gap in the armour, a weakness that can be attacked instead.
The origins of this proverb is from the Warring States Period. The state of Wèi attacked Zhao and laid siege to its capital Handan. Zhào turned to Qí for help, but the Qí general Sun Bin determined it would be unwise to meet the army of Wèi head on, so he instead attacked their capital at Daliang. The army of Wèi retreated in haste, and the tired troops were ambushed and defeated at the Battle of Guiling, with the Wèi general Pang Juan slain on the field. Note that this campaign is also described explicitly in the Art of War of Master Sun Bin the younger.
The idea here is to avoid a head on battle with a strong enemy, and instead strike at his weakness elsewhere. This will force the strong enemy to retreat in order to support his weakness. Battling against the now tired and low-morale enemy will give a much higher chance of success.
本指围攻魏国的都城以解救赵国。现借指用包超敌人的后方来迫使它撤兵的战术。作者: HoCheung 时间: 2008-8-29 10:31 标题: 学说英语“三十六计”——瞒天过海 Deceive the heavens to cross the ocean
The stratagem references an episode when Tang emperor Tang Gaozong balked from crossing the sea to a campaign against Koguryo. His General brought him the invitation of a wealthy peasant to supper. As the emperor entered this house, it began to shake - the General had deceivedly lured the emperor onto a ship! Upon discovering this, the emperor decided to carry on and later finished the successful campaign.
This stratagem means that you mask your real goals, thus deceiving even the emperor / heavens itself with a fake goal, until they are confronted with finished facts. Harro von Senger notes in the German-Language "Die List" that to grasp the full meaning, it would be something like "to deceive the holy virgin Mary" in the western hemisphere.
This strategy made use of the human weakness that, people eventually become unaware of common everyday activities, or events that keeps repeating itself. When this happens, it is the best moment carry out one's previously hidden true objective.