The Sages taught in a Tosefta that one who circumcises a child recites: “Who has made us holy through His commandments, and commanded us concerning circumcision.” The father of the circumcised child recites: “Who has made us holy through His commandments, and commanded us to bring him into the covenant of Abraham, our father.” Those standing there recite: “Just as he has entered into the covenant, so may he enter into Torah, marriage, and good deeds.”
And the one who recites the additional blessing says: “Who made the beloved one holy from the womb, marked the decree in his flesh, and gave his descendants the seal and the sign of the holy covenant. Therefore, as a reward for this, the living God, our Portion, commanded to deliver the beloved of our flesh from destruction, for the sake of His covenant that He set in our flesh. Blessed are You, Lord, Who establishes the covenant.”
The one who recites the blessing is a designated individual, whether the person who performed the circumcision or any other person, who holds a cup of wine in his hand and recites an additional blessing over the circumcision. Some authorities explain that the rationale for the second blessing is that since this mitzva was already fulfilled by the Patriarchs, the Sages instituted a special blessing in their honor (Tosefot Rid).
Regarding the additional prayer that is added Tosafot explain that the statement “Who made the beloved one holy from the womb” alludes to all of the Patriarchs. Some commentaries suggest that Rashi says this entire blessing refers only to Yitzhak, because he was the first to be circumcised at eight days. Yitzhak is described as beloved in the verse “Your son…whom you love” (Bereshit 22:2; Me’iri). There is also a dispute over the word tzadi, vav, heh that appears in this blessing: Some commentaries explain that it should be pronounced tzavei, and it is a request that God save us from the punishments of Gehenna. That reading appeared in many prayer books.
The ge’onim and others, however, read it as tziva, past tense, in praise for an event that already took place.