It is a common myth that Gandhari took away the cloth around her eyes and looked at Duryodhana calling upon her tapo-bal and made his body invincible except for the thighs, which he had covered. But this is not true.
The reason for Duryodhana’s upper body being strong, lower being weaker is told to us after the ghosha yatra incident.
Karna plans that they should all go to Dwaitavana to show off to the Pandavas how they, the Kauravas, are living in luxury and comfort. But at Dwaitavana, Gandharvas capture Duryodhana, his brothers and his wives. Karna flees from the battle. Duryodhana and his people are rescued by Arjuna and the other 3 brothers.

When Duryodhana finally gets free, he is completely depressed that he owes his live to the Pandavas. Karna finds him the next morning and Duryodhana says that he will stay right there and die because he does not wish to live a life gifted by the Pandavas. Karna, Dushasana and Shakuni al convince him not to do that and to come back to Hastinapur.
But a dejected Duryodhana sits in a trance, deciding to give up his life. The Daityas and danavas are anxious now because they are counting on Duryodhana to fight for evil, help the danavas who have been born on earth. They create a being from the fire who goes and brings Duryodhana to the underworld. They tell him that they had ensured his birth by getting a boon from Mahadeva. His body was created by Mahadeva and Parvati devi- the upper part from Mahadeva, strong as vajra, the lower by Devi, beautiful as flowers. This is the reason why Duryodhana’s upper body is strong but lower body can be penetrated.