In the third berachah of the Amidah, Kedushah is recited. The essence of the Kedushah is in the responses, “Kadosh, kadosh, kadosh, Hashem Tzevakot, melo kol ha’aretz kevodo” (“Holy, holy, holy is Hashem, master of Hosts, the whole world is filled with His glory”) and “Baruch kevod Hashem mimkomo” (“Blessed is the glory of Hashem from His place”). Originally, it was customary for only the chazan to recite the connecting passages and the congregation would answer the verses, “Kadosh,” “Baruch,” and “Yimloch” (Shulchan Aruch 125:1). Nowadays, according to the Arizal, the congregation also recites the connecting passages, the chazan repeats them aloud afterwards, and the congregation responds with the verses of the Kedushah (Mishnah Berurah 125:2; Kaf HaChaim 2).
Some say that the verse “Yimloch” is not an essential part of Kedushah, but rather one of the chazan’s connecting passages, and therefore, if a person is in the middle of reciting Birkot Keriat Shema, and he hears Kedushah, he may only recite the verses “Kadosh” and “Baruch” and not the verse “Yimloch.” Others say that he recites “Yimloch”, since this verse is included in Kedushah as well. This is the prevalent minhag.
Kedushah is recited a total of three times in Shacharit: in Birkat Yotzer Or, in Chazarat HaShatz, and in the passage U’va L’Tzion. However, the poskim disagree as to whether the laws of the Kedushah in Chazarat HaShatz apply to the other two, and whether it is necessary to recite them in a minyan. In practice, the halachic ruling is that an individual is permitted to recite them. However, to avoid uncertainty, it is best that he recite them in a melody of cantillation as if reading from the Torah (see further in this book 16:4; 23:2). The Kedushah in the Amidah repetition is the essence of Kedushah and it is only recited with a minyan.
It is proper to stand with one’s feet together for Kedushah, since we recite this Kedushah like the angels whose legs are so close together that they resemble one leg (Shulchan Aruch 125:2). There are those who enhance the mitzvah by remaining with their legs together until the end of Birkat HaKel HaKadosh (Eliyah Rabbah 125:6). However, this is not an obligation and many prominent rabbinic authorities are not strict concerning this.
It is customary to raise one’s heels slightly when saying “Kadosh Kadosh Kadosh,” “Baruch,” and “Yimloch,” and to turn one’s closed eyes upward, thereby expressing the desire to transcend physical boundaries and soar upward. The source for this resides in the Midrash (Beit Yosef and Rama 125:2; Mishnah Berurah 6; Kaf HaChaim, paragraphs 2 and 9).