The Shulchan Aruch (Orach Chayim 56:4; listen to audio for direct citation) lists four points during the Kaddish recitation at which the Chazan (one who leads the services) must bow. They are: when reciting "Yitgadal," which is the first word of Kaddish; then when reciting the word "Rabba" in the phrase "Yehei Shemei Rabba"; then when reciting "Berich Hu"; and lastly when reciting "Amen" in the phrase "Da'amrian Be'alma Ve'imru Amen."
Additionally, The Kaf Ha'chayim (by Rabbi Chayim Palachi, Rabbi of Izmir, Turkey, 19th century) cites from a number of Poskim that one should also bow when reciting the word "Yitbarach." Rabbi Mazuz, in his responsa "Ish Matzliach," codifies this ruling of the Kaf Ha'chayim, and this is indeed the procedure written in many of our Siddurim.
Thus, there are five points during the Kaddish recitation when one must bow:
1) The opening word "Yitgadal."
2) The word "Rabba" in the phrase "Yehei Shemei Rabba."
3) The word "Yitbarach."
4) The words "Berich Hu."
5) The word "Amen" in the phrase, "Da'amiran Be'alma Ve'imru Amen."
At times Chazanim are preoccupied as they recite Kaddish thinking about maintaining the proper speed or singing a melody, and as a result they forget to bow at these points in the Kaddish. One who serves as Chazan must therefore keep in mind this requirement as he recites the Kaddish.
