The Gemara in Masechet Berachot records a comment by Eliyahu Ha'navi that whenever Jews assemble in a synagogue and declare "Yehei Shemei Rabba Mevarach" in the Kaddish, the Almighty exclaims, "Fortunate is the King whose children praise Him such in His home!" Upon hearing this declaration God is reminded of the Beit Ha'mikdash, where the Kaddish was recited, and this is therefore a moment of love and compassion for the Jewish people. The Gemara further comments that if one answers "Yehei Shemei Rabba" with all his might – which means either with a loud voice or with full concentration – any harsh decrees that had been issued against him are revoked. This demonstrates the immense importance the Sages afforded to the Kaddish.
The words "Yehei Shemei Rabba" mean "May His Name become great." Based on a verse in the Book of Shemot (17:16 – "Ki Yad Al Keis Y-H, Milchama Le'Hashem Ba'Amalek"), the Rabbis comment that God's Name is complete only when Am Yisrael's enemies are defeated. In the Kaddish service we thus pray for the day when we will triumph over and subjugate our enemies, so that God's Name will finally become complete and exalted throughout the world.
The scholars of Kabbala have identified a number of different mystical allusions in the Kaddish prayer. For example, the phrase "Ve'yishtabach Ve'yitpa'ar Ve'yitromam Ve'yitnasei Ve'yit'hadar Ve'yit'aleh Ve'yit'halal" consists of seven words, each of which begins with the letter "Vav," which has the numerical value of six. The product of seven and six is forty-two, and thus this phrase alludes to the 42-letter Name of God. By the same token, each of these seven words consists of six letters, providing yet another allusion to the 42- letter Name.
The following is a list of some of the basic laws relevant to Kaddish:
1) One must ensure not to speak during the recitation of Kaddish. The Mishna Berura (commentary to the Shulchan Aruch by Rabbi Yisrael Kagan, the "Chafetz Chayim," Lithuania, 1835-1933), in Siman 56, records several accounts of otherwise righteous men whose souls were blemished in the afterlife due to their having spoken during Kaddish. One should likewise refrain from learning Torah or thinking of any other matters during Kaddish, focusing his attention instead on the words of Kaddish.
2) Our custom is to follow the practice of the Arizal (famed Kabbalist, Israel, 1534-1572) and respond to Kaddish with twenty-eight words, from "Yehei Shemei Rabba" through the words "Da'amiran Be'alma." One must ensure to proper pronounce all twenty-eight words, and should pay particular attention to enunciate the word "Min" in the phrase "Le'eila Min Kol Birchata." Similarly, the words "Berich Hu" must be pronounced as two separate words, and not combined into a single word.
3) Some people mispronounce the word "Yitgadal" as "Yitkadal," which has a negative connotation. One must ensure to recite the word properly. There are those who also mispronounce the word "Ve'sheizava" as "Ve'sheizafa." One must accustom himself to reciting these words properly.
4) According to Sephardic custom, it is permissible to sit during the recitation of Kaddish unless one had been standing when the Kaddish began, in which case he must stand until the words "Da'amiran Be'alma." After one recites these words, he may sit, even if the Chazzan has yet to reach that point. Ashkenazim, however, have the practice of standing for Kaddish even if one had been sitting when Kaddish began.
