The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Tesave, writes that the Hazan announces "Kohanim" to prompt the Kohanim to begin their Beracha. Birkat Kohanim takes place during the Hazan’s repetition, immediately after the Beracha of "Ha’tob Shimcha U’l’cha Na’e Le’hodot." After the congregation completes its "Amen" response to this Beracha, the Hazan should announce "Kohanim" to signal to the Kohanim to begin their blessing. The Hazan should not make this announcement before the congregation completes their response of "Amen," and the Kohanim should not begin reciting the Beracha before the Hazan makes his announcement.
The Hazan announces "Kohanim" only if there are at least two Kohanim present in the synagogue for Birkat Kohanim. If there is only one Kohen, the Hazan cannot announce "Kohanim," which is a plural term, and it is not customary to announce "Kohen" in the singular form. The custom in our community is that in such a case, somebody from the congregation besides the Hazan announces "Emor," which is based upon the Torah’s formulation in introducing the Misva of Birkat Kohanim ("Amor Lahem" – Bamidbar 6:23). The Hazan should not announce "Emor," as this would constitute a Hefsek (forbidden interruption) in his repetition of the Amida (as opposed to announcing "Kohanim," which is, in a certain sense, part of the Amida prayer). After the announcement of "Emor" is made, the Kohen begins the Beracha of Birkat Kohanim.
This is an important Halacha for everybody to know. The Kohen does not begin Birkat Kohanim until after he hears "Emor," and the Hazan, as mentioned, cannot make this announcement, and therefore the congregants must be aware of this Halacha so that somebody will announce "Emor" to prompt the Kohen. Only one congregant should make this announcement.
If no Kohanim are present, then at the point in the Amida when Birkat Kohanim is normally recited, the Hazan recites a special insert that begins "Elokenu V’Elokeh Abotenu Barechenu Ba’beracha Ha’meshuleshet…" This insert cites the verses of Birkat Kohanim. After the Hazan recites each verse, the congregation should respond, "Ken Yehi Rason." They make this response after each verse – after "Ve’yishmerecha," after "Vi’huneka," and after "Ve’yasem Lecha Shalom." They do not respond "Amen" after each verse as they do when the Kohanim recite Birkat Kohanim. The Hesed La’alafim (Rabbi Eliezer Papo, 1770-1828) records a custom observed by the "Vatikim" (pious individuals) to respond not just "Ken Yehi Rason," but "Ken Yehi Rason Bi’zchut Abraham" after the first verse, "Ken Yehi Rason Bi’zchut Yishak" after the second verse, and "Ken Yehi Rason Bi’zchut Yaakob, Moshe, Aharon, Yosef Ve’David" after the third verse.
Summary: After the Hazan concludes the Beracha of "Ha’tob Shimcha U’l’cha Na’e Le’hodot" in the repetition of the Amida, he waits for the congregation to answer "Amen" and then announces, "Kohanim." At that point, the Kohanim begin Birkat Kohanim. If there is only one Kohen present, then instead of the Hazan’s announcement of "Kohanim," somebody else from the congregation announces, "Emor." If there are no Kohanim at all, then in place of Birkat Kohanim the Hazan recites the insert of "Elokenu V’Elokeh Abotenu Barechenu," which cites the text of Birkat Kohanim, and after each verse of the blessing the congregation responds "Ken Yehi Rason," and not "Amen."
