Kedusha is recited in the third berakha of Ĥazarat Ha-shatz. The essence of the Kedusha is the congregation’s response with the verses: “Kadosh, kadosh, kadosh, Hashem Tzevakot, melo kol ha-aretz kevodo” (“Holy, holy, holy is the Lord of hosts; the whole world is filled with His glory”; Yeshayahu 6:3), “Barukh kevod Hashem mi-mkomo” (“Blessed be the Lord’s glory from His place”), and “Yimlokh Hashem Le-olam…” (“The Lord shall reign forever…; Tehilim 146:10). The custom today, based on Arizal, is for the congregation to recite the connecting passages as well. The ĥazan then repeats them aloud, and the congregation responds with the verses of the Kedusha (MB 125:2; Kaf Ha-ĥayim 2).
It is best to stand with one’s feet together for Kedusha, since we recite this Kedusha like the angels whose legs are so close together that they resemble one leg (SA 125:2). There are those who beautify the practice by remaining with their legs together until the end of Ha-Kel Ha-Kadosh (Eliya Rabba 125:6); however, this is not an obligation.
It is customary to raise one’s heels slightly and turn one’s closed eyes upward when reciting the words “Kadosh” (all three times), “Barukh,” and “Yimlokh,” thereby expressing the desire to transcend and soar upward (Beit Yosef and Rema 125:2; MB 6; and Kaf Ha-ĥayim 2 and 9, which state that some people keep their eyes open).
When the ĥazan reaches Modim, the whole congregation bows with him and recites Modim De-rabanan, whose nusaĥ differs from that of the Modim in the Amida, as clarified in the Talmud (Sota 40a). This bow must be in accordance with the laws of Modim in the silent Amida (MB 127:2; Kaf Ha-ĥayim 1; see above, 12:5).