How is it possible to declare a person Cherem?
Rashi: It refers to one's Avadim and Shefachos Cana'anim.
What is this Pasuk coming to teach us?
Rashi (from Erchin, 6b): It is teaching us that if Reuven, who has been sentenced to death at the hand of Beis-Din, is being taken out to be killed, and Shimon declares 'Erko Alai!', his declaration is ineffective.
Ramban (citing Erchin, 6b) and Rashbam: It teaches us that a person who has been sentenced to death by Beis-Din cannot be redeemed.
Ramban: With reference to a declaration by the Jewish army that in the event that Hashem delivers the enemy into their hands, they will be Machrim their cities, 1 this does not mean that all the contents, including the inhabitants, should be given to the Kohanim (or should be Hekdesh), 2 but that the entire town must be destroyed and all that it contains, Asur b'Hana'ah. 3
Ramban (citing Raboseinu): It is Lav she'biChelalos, 4 incorporating all of the above (refer to 27:29:2:1-3), in addition to placing the death-penalty upon someone who transgresses a Cherem imposed by the Beis-Din ha'Gadol or by the king. 5
Ramban: Because the Din of Cherem as regards Adam, only applies to Avadim who are his personal property, but not to people who are not. In fact, this was the mistake that Yiftach made, in thinking that the Neder that he made declaring the first one to come and greet him after his victory would be Hekdesh, pertained to his daughter as well (See Ramban DH 'v'Zeh' and 'v'Al Tihyeh'..
Ramban: Since that is obviously what they meant.
Ramban: Like the Lav of "Lo Sochlu al ha'Dam" ( 19:26), and that of "Lo Yumsu Avos al Banim" - Devarim 24:16, as the Gemara explains in Sanhedrin, 27b.
Ramban: As we find in the case of Yonasan, son of Shaul, whose father initially sentenced him to death for contravening the Cherem that he had imposed upon the army - See Shmuel 1. 14:45.
What does "Mos Yumas" mean?
Rashi: It means that because he is being taken out to be killed, he has no value and no Erech, and cannot therefore be redeemed. 1
Ramban: Refer to 27:29:2:3.
For his value to be given to the Kohanim or to Bedek ha'Bayis.
Rashi writes that if one is being taken out to be killed, and another is Ma'arich him, it has no effect. What is the reason? If we find a reason to acquit him, we return to judge him again!
Riva citing R. Tam of Orlins: This is why a verse is needed to exempt. (One might have thought that he has an Erech, for perhaps he will be acquitted.)