After describing the choshen and the stones which adorned it, the Torah describes the Urim v'Tumim: “You shall put in the breastplate of judgment the Urim and the Tumim; and they shall be on Aaron’s heart, when he goes in before Hashem: and Aaron shall bear the judgment of the children of Israel on his heart before Hashem continually.” (Shemos 28:30). What are these things and what do they do?
According to Ramban (ibid.), the Urim v’Tumim are holy Names of God, not known to anyone except to the wisest of Israel, and they transform the choshen into a means of communication between God and man. The Names of God called Urim illuminate the letters which compose God’s message. The letters all light up simultaneously, and the Kohein Gadol must unscramble them. The Names of God called Tumim perfect the knowledge of the Kohein Gadol, enabling him to understand their meaning.
The greater the human agency in communicating with God, the more perfect the communication. Thus, prophecy, which requires man’s perfection and resembles conversation, is the highest form of communication. A bas kol — a divine voice emanating from the Heavens for all present to hear — is the lowest because man is entirely passive in its receipt. Communication through the Urim v’Tumim, therefore, lies in between these modes. (ibid.).
As an illustration, Ramban cites the first chapter of Shoftim. (See also Yoma 73b). B’nai Yisrael were unsure who to send to war with the Cananites. Yehoshua had died — Israel lost its prophet — so they relied on the inferior guidance of the Urim v’Tumim, which instructed them to send Shevet Yehudah to war with King Adoni-bezek. After defeating his army, B’nai Yisrael barbarously and cruelly cut off Adoni-bezek’s toes and thumbs. Adoni-bezek claimed he deserved it, for he had committed the same barbarous and cruel acts against his own enemies. Maybe so. However, knowing your own barbarity is accidental recompense for the barbarity of your victim is little relief. Indeed, if B’nai Yisrael still had a man at their helm, instead of stones on a breastplate, they may have conducted a just war.