There are four halachot specified in the Torah concerning Birkat Kohanim: “It is only recited in the Holy Tongue, while standing, with raised hands, and in a loud voice” (Shulchan Aruch 128:14). When one of these conditions cannot be met, the Kohanim may not bless the nation.
The first halachah requires that the berachah be recited in Hebrew, the Holy Tongue. Many mitzvot are performed through speech. The Chachamim explained in the Mishnah (Sotah 32a) that the mitzvot of Keriat Shema, Amidah, and Birkat HaMazon (Grace After Meals) can be fulfilled in foreign languages (see 1:10; 16:9). Nevertheless, Birkat Kohanim must be recited specifically in the Holy Tongue, as it is written (Numbers 6:23), “This is how you must bless,” meaning, in the language in which the Torah is written.
A possible explanation for this is that the purpose of Keriat Shema is to express the foundation of our faith, and therefore the particular language in which it is recited is less important than the understanding of what we are saying. Likewise, regarding the Amidah, the essence is that we convey our prayer in a comprehensible manner. However, Birkat Kohanim is a berachah from Hashem to us, and the language in which Hashem reveals His will in the world is the Holy Tongue. Therefore, the Kohanim are commanded to recite the Divine blessing precisely as it is written in the Torah.
The second halachah is to recite Birkat Kohanim while standing. Therefore, a weak or handicapped Kohen who must sit in a wheelchair, and cannot stand on his feet even while only reciting the blessing, may not bless the people. This is because the law of reciting Birkat Kohanim is similar to the law of the Temple service, as it is written (Deuteronomy 10:8), “To serve Him and offer blessing in His Name.” Just as the Temple service is performed while standing, so too, the berachah must be recited while standing (Sotah 38a).
A person presents himself fully in a standing position, for in that way he is seen from head to toe, expressing his complete range of abilities, both spiritual and practical. In order for the work of the Kohanim and Birkat Kohanim to be fulfilled perfectly, they must be carried out specifically while standing.
The third halachah obligates the Kohanim to raise their hands while reciting the berachah, meaning that they must spread their hands out over the people being blessed, as it is written (Leviticus 9:22), “Aharon lifted his hands toward the people and blessed them.” A Kohen whose hands tremble and are weak, and who does not have the strength to hold his hands up during the berachah, is not permitted to ascend the duchan and bless. Even if he is helped in raising his hands, it would be of no use, since the Kohen must raise his hands by himself without any assistance (Mishnah Berurah 128:52).
Rabbi Nachman of Breslav offers a spiritual explanation of this halachah, stating that the lifting of one’s hands conveys the willpower of the heart to bless Israel with love. There is a connection and relation between one’s hands and one’s heart, the proof being that a person’s hands are close to his heart. Therefore, hand motions are the heart’s instruments of expression, as it is written (Lamentations 3:41), “Let us lift up our hearts with our hands to God in Heaven.” When the Kohanim stretch their hands out to bless Israel, they are thereby expressing their heart’s great love towards Israel (Likutei Halachot Nesiat Kapayim 5:3).
Rav Kook adds that raising one’s hands forward symbolizes the future, for, indeed, the hands transcend beyond the body. Therefore the Kohanim lift their hands to convey the yearning and prayer for an ideal and rectified world (Olat Ra’ayah, part 1, 284).
The fourth halachah commands the Kohanim to recite the blessing aloud so that all the people praying in the synagogue can hear them, as it is written, “Say to them,” meaning, in the manner that a person talks to his friend (Sotah 38a). If the synagogue is small, it is sufficient for the Kohen to recite the blessing in an average tone, for that is how people normally talk. If the synagogue is large, he must recite the blessing loud enough for everyone to hear him, even if he were to recite it by himself. A Kohen who has such a faint voice that it is almost impossible to hear him does not ascend to recite the blessing (Mishnah Berurah 128:53). However, if there are other Kohanim present whose voices will be well heard, it is permissible for the Kohen with the weak voice to ascend with them and bless (see Tzitz Eliezer 15:21).