Migu is a Rabbinic concept in which a witness establishes his reliability by his choice of defenses. It goes like this: a claimant has a sure-win claim, but instead offers an alternative defense which, on its own, we would not believe but for the fact that he could have chosen the sure-win claim. We say to ourselves, "if this guy was lying, he would have chosen a better argument."
Many cases come to mind. In Kesuvos 12b there is a case where a woman claims she lost her signs of virginity when she was raped -- she is believed because she could have claims that she lost the signs due to an accident (and thereby retain her right to a 200 zuzim value to her kesubah; because she claimed to be raped, she only will get 100 zuzim).
At Bava Basra 31a, Shimon claims to own land and Reuven disputes. Shimon has a migu claim: He says that he has lived on the land for 3 years and thereby acquired the land. He could have claimed that he had bought the land from Reuven's father and, although he has no receipt, his possession of the land for 3 years supports his claim. By making the lesser argument, we assume that he's telling the truth. However, in the Bava Basra case, Reuven trumps Shimon by bringing two witnesses. Migu is good; witnesses are better.
A friend of mine, a convert, was living in Jerusalem and the rabbis were reluctant to accept his conversion because the rabbis on the convert's bet din were not on the official government list of approved conversion rabbis. Converts in Israel are questioned because conversion grants one the capability of making aliyah with full immigration rights. Therefore there is incentive for an insincere person to "convert." But my friend was able to bring proof that he had divorced his wife by giving her a get. They accepted that as a migu. He could have claimed to have been born Jewish and would not have been easily disputed. But the fact that he admitted to giving his wife a get and that he was a convert demonstrated that he was a ger tzedek because an insincere convert would not have bothered to divorce his wife with a get.