The Gemara in Masechet Shabbat (119) teaches that responding "Yeheh Shemeh Rabba" to Kaddish "with all one's strength" has the power to annul harsh decrees. According to some versions of this passage, even if a decree of seventy years of suffering was issued against a person, he can have the decree repealed by answering "Yeheh Shemeh Rabba" with all his "strength."
The common understanding of this expression – "with all one's strength" – is that it refers to full Kavana (concentration). Answering to Kaddish with concentration, focusing on the meaning of the words, has the power to annul harsh decrees.
People often look for effective "Segulot," especially when they are dealing with some kind of problem or crisis, or when they have an important court case or business deal. Unfortunately, they generally overlook what might be the most obvious and most well-documented "Segula" of all – responding to Kaddish with full concentration. No matter what harsh punishment has been decreed against a person, he has the opportunity to have it annulled by responding to Kaddish properly.
People who talk during Kaddish need to remember that they can gain far more by concentrating during Kaddish than they do with any conversation they have with their fellow. The "return on investment" for properly concentrating during Kaddish is far greater than we could ever imagine. The Yeser Ha'ra (evil inclination), knowing the great benefits of concentrating on Kaddish, lures a person to disregard Kaddish, and to engage in conversation instead of listening and responding properly. But speaking during Kaddish – even words of Torah! – is strictly forbidden by Halacha, and by doing so, one forfeits the immense rewards that this special prayer offers, and becomes liable to punishment, Heaven forbid. The Bet Yosef brings the story of Rabbi Hama who saw Eliyahu Ha'nabi leading thousands of camels loaded with "anger and wrath," and Eliyahu said that all this anger is for those who engage in conversation during the recitation of Kaddish. And the Kaf Ha'haim (Rav Yaakob Haim Sofer, Baghdad-Jerusalem, 1870-1939) cites the Mateh Moshe as relating that a certain Torah scholar had a student who died young, and the student appeared to him in a dream, with an unseemly mark on his forehead. The student explained that this mark was his punishment for speaking during Kaddish.
One should not fold his Tallit or Tefillin, or engage in other activity, during the recitation of Kaddish, so that he can fully concentrate on the words. This applies to all the Kaddish recitations – the Kaddishim recited during the prayer service, the Kaddish recited after Torah learning, the Kaddish recited at an Arayat, and so on. Rav Yisrael Bitan cites an opinion that this applies only when one responds, "Yeheh Shemeh Rabba," though Rav Bitan disagrees, and maintains that this is forbidden even while listening to Kaddish. He adds, however, that this is forbidden only through "Da'amiran Be'alma," which is the essential Kaddish. During the remainder of Kaddish, which is a later addition to Kaddish, it is permissible to engage in other activities.
If someone fell behind during the prayers, and needs to complete the previous prayer during Kaddish, he should do so only after "Da'amiran Be'alma." Until that point, he should remain silent and respond to the Kaddish. Rav Bitan cites this ruling from the Mishna Berura.