Everyone was determined, "I must kill." Duryodhana was determined to kill Bhima, and Bhima was determined to kill. So when they did not listen to the instruction of Baladeva, then Baladeva left the place. He went to Dvaraka, back. Of course, Krishna was there, because Krishna was to give hint to Bhima how to kill Duryodhana. So Duryodhana, by the blessings of his mother, his whole body became iron-like solid. So by beating by the Bhima, it was not easy.
So the story is that Gandhari, the mother of Duryodhana, she was great chaste woman. Because her husband was blind, she used to remain as blind woman covering the eyes. But she had some power. Chaste woman, anyone who sticks to the regulative principles, he gets a power, spiritual or material. He gets power. A brahmacari gets power if he follows brahmacarya. Everyone, if we follow the prescribed rules and regulations, automatically you become powerful.
So Gandhari had some power. So her eldest son, Duryodhana, was advised to see the mother naked. She advised, "My dear son, tomorrow morning when you come to offer your obeisances to me, you come naked. I shall see you and you will be solidified just like iron." So he was going naked and Krishna saw. So He asked him, "Where you are going?" "I am going to see my mother." "How is that? You are going naked? At least you have some langota. This is not good." So he took the instruction of Krishna and covered the private part with a langota.
And when Gandhari saw, she saw that he was not fully naked, so she regretted, "O my dear son, I asked you to come before me naked. Why you have got this...?" "No, Krishna advised." Then she began to smile, that, "My attempt is failure." So Krishna knew it, that part which was covered, that was not turned iron. The other parts turned into iron on account of seeing by Gandhari.
So in this fight Krishna hinted Bhima that, "You strike here. That part is not ironized." Although it is against the regulative principle to strike the opponent party below this waist, Krishna advised that, "Unless you transgress this law, you cannot kill him." So he was stroken below the waist, and he was not killed, but his waist was broken. Therefore it is said vrikodaraviddha-gadabhimarsa. Then he died.