Drona has a few questions for Duryodhana, 'You asked me to only capture Yudhisthira. He is surely fortunate! Why is it you did not ask for his death? Is it because you wanted to spare me from killing him. Or is it that Dharmaraja has no enemies. Or you wanted that at least one of them to remain to propagate their vamsha. Or you wanted to win and then share the kingdom with them, be friends again and have peace. Whatever it is Yudhisthira is fortunate! With your show of friendship, and thus becoming अजातशत्रु , one without enemies, his life has truly become meaningful.'
These words from Drona takes Duryodhana unawares and has the effect of exposing his real intentions! It does happen that even the most intelligent cannot sometime hide what is on their minds. Duryodhana replies with a smile 'If we kill Yudhitshira, we will never win the war. His brothers will surely destroy us. Even gods cannot defeat them. If we capture him, we can again defeat him in a game of dice and send him to the forests. His brothers will then follow him. It is as good as a win. This is the reason I did not want him killed!'
Drona understands Duryodhana's devious intentions and adds a condition to the boon he granted. 'If Arjuna does not protect Yudhisthira, consider that he is in your hands. But if Arjuna comes to his rescue, even Indra cannot face him. He may be my student, but he is very young, is blessed with many astras from Indra, Rudra and such. So I would not like to fight him. If you manage to take him away some where, you have Yudhisthira'. Duryodhana agrees to this condition and imagines Yudhisthira already in his captivity!