The Kadish of the Yamim Noraim (High Holy Days) includes the special "Sha’are" prayer to open all of the Heavenly gates, to which everyone answers Amen. The only problem is that people often emphasize this part by answering in a loud strong voice, whereas they respond to the primary phrase of the Kadish, "Yeheh Shmeh Rabah…", softly and weakly. This is backwards. The Gemara states that whoever answers "Amen, Yeheh Shmeh Raba…" with all of his strength and Kavana, even if he has a negative judgment of seventy years, it is ripped up. Clearly, this phrase has a greater effect than the "Sha’are" does.
The power of answering Kadish is illustrated by the following account found in Pirke Hanoch: The angel told Rabbi Yishmael that he would reveal to him what is destined for the Jewish people. He took him into the inner chambers of the treasury. There, were all the ledgers, inscribed with many misfortunes and troubles. Rabbi Yishmael asked the angel, "For who are these troubles?" He answered, "For the Jewish people." The Rabbi asked, "How can they bear them?" The angel replied, "Come tomorrow and I will show you a new list of even greater troubles." The next day he brought him in again and showed him the troubles written there, which included war, famine and captivity. The angel said that each day new and severe punishments are decreed on the Jewish people. However, when they come to the Bet Knesset and answer "Amen Yeheh Shmeh Rabah…" to the Kadish, they suppress these decrees from leaving the inner chamber. Thus, answering the Kadish properly saves us from many harsh decrees that we are not even aware of.
Especially during the High Holy Days, on should make an extra effort to answer "Amen Yeheh Shmeh Rabah" with all of his strength and Kavana.