Is there an idiom in English for this meaning?

I don't think so. 'Malinger' means to pretend injury or sickness to avoid military service, but that's fooling your superiors, not the enemy. Maybe you want 'to play possum' - which means to pretend you're dead in the presence of enemies. (Possums, or, more correctly, Opossums, are animals that curl up and lie still when a predator catches them in the hopes that the predator will lose interest in prey that is already dead.)

Hi Eos. Thanks for reply.

But what you introduced is not what I wanted to know.

Here is an example explaining what I meant.

There was in history General A led his troops to attack the troops led by General B, but the former could not defeat the latter for long. For breaking the ice, a lieut called Jack who was absolutely loyal to General A, advising General A a secret plan -- Jack cut off his left arm with his own knife and, regardless of General A's dissuading him, in the night he pretended to flee away from his troops to the General B's camp, and showed his broken arm to General B, saying with tears in his eyes:"I have a very beautiful wife, but General A has been planning grabbing her. Today, he once again wanted me give up my wife to him, of course I refused him in no time. So he cruelly ordered to cut off my left arm." And then, he added:"I come here to ask you help me take vengeance." Hearing this and seeing his broken left arm, General B and his following were scared and tearing. They called him "Bitter Man", and letting him go around their camp to tell his miserable story to all soldiers -- because they thought his story could fuel hatred and raise morale against General A and his troops. I think you understand what had happened afterwards. Jack got the information about General B and his troops and disappeared in the camp and reappeared before the General A.As a result, General B suffered his Waterloo, his troops routed as hell.

So we call that secret plan Jack advised General A as "Bitter Man Plan", a plan that injures one's self to win the confidence of his enemy.

Of course, I don't know if it is proper we call this "Bitter Man Plan", that is why I wanted to ask you.

(Please revise the story above if you feel there are some imporper grammars)

I don't think there is an expression in English which would exactly correspond to this, and certainly no idiom.

Using a "decoy" is maybe the nearest thing, when you try to trick an enemy by pretending things are different than they actually are.

I am afraid the decoy seems too vague for this.

But thanks for reply.

I found out New York Times also could not find the idiom in English, but just using Chinese Pinyin "ku rou ji" instead.

Some Nationalist supporters said the incident reminded them of a "ku rou ji," using an ancient Chinese term for a self-inflicted wound intended to trick a foe.

Click the link below to check out the news in NYT that contains "ku rou ji".

http://www.nytimes.com/2004/03/20/international/asia/20TAIW.html?th

Around these parts we refer to it as being an "inverse Benedict Arnold."