Birchat Kohanim: Don't Look!

(22) The LORD spoke to Moses: (23) Speak to Aaron and his sons: Thus shall you bless the people of Israel. Say to them: (24) The LORD bless you and protect you! (25) The LORD deal kindly and graciously with you! (26) The LORD bestow His favor upon you and grant you peace! (27) Thus they shall link My name with the people of Israel, and I will bless them.

Who is this Mitzvah for exactly?

(1) Birkat Kohanim is not performed if there are fewer than ten, including the Kohanim in the quorum. Ram"a: A non-kohen should not perform Birkat Kohanim, even if there are other kohanim present (Ketubot, ch. 2, daf 24, states that a non-kohen violates a positive commandment; but Tosafot in the chapter "Kol Kitvei" (Tr. Shabbat) states that R"i does not know what prohibition there would be for a non-kohen who ascends [for Birkat Kohanim], and it is possible that with other kohanim it would be permitted; but this requires further consideration).

The Gemara suggests: Come and hear proof from a baraita: Presumptive status for priesthood is established by the lifting of hands in Babylonia; by partaking of ḥalla in Syria... In any event, the tanna teaches that the lifting of hands establishes the presumptive status of priesthood. ... The Gemara suggests: Come and hear proof from a baraita: Presumptive status for priesthood is established in Eretz Yisrael by the lifting of hands and distribution of teruma at the threshing floors. And in Syria and everyplace outside Eretz Yisrael that emissaries informing residents of the Diaspora of sanctification of the New Moon arrive, the lifting of hands constitutes proof of presumptive status for priesthood, as the court would investigate the lineage of everyone who recited the Priestly Benediction.

Rabbi Yehuda, son of Rabbi Naḥmani, the disseminator of Reish Lakish, interpreted a verse homiletically: Whoever looks at the following three things, his eyes will grow dim: One who looks at a rainbow, at a Nasi, and at the priests....The third item, looking at priests, is referring to one who looks at the priests when the Temple is standing, as they would stand on their platform and bless Israel with the ineffable name.

They would bless Israel with the ineffable name, at which point the Divine Presence would rest above the joints of their fingers.

MISHNA: A priest who has blemishes on his hands may not lift his hands to recite the Priestly Benediction. Rabbi Yehuda says: Even one whose hands were colored with satis, a blue dye, may not lift his hands to recite the Priestly Benediction because the congregation will look at him.

(23) At the time that the kohanim bless the congregation, they should not glance [around] nor get distracted; rather, their eyes should face downward in the same way one stands in prayer. The congregation should be attentive to the blessing, and their faces should face the kohanim, but they should not stare at them. [Ram"a: And the kohanim should also not stare at their own hands; therefore, it is customary for them to wrap their tallis on their faces and keep their hands outside the tallis. And there are some places where they have the custom that their hands are kept within the tallis, so that the congregation does not stare at them (Bet Yosef).]

(35) A kohen who has killed a person, even unintentionally, may not perform the priestly blessing, even if he has repented. Ram"a: Some say that if he has repented, he may perform the blessing, and there is ground to be lenient with respect to the penitent, so as not to shut the door before them. And so is the custom (his own opinion, because this is not worse than a [repentant] apostate) (Tur, Rashi, many others, Agur, Bet Yosef).