Three conditions must be present for an individual to fulfill his obligation by hearing Ĥazarat Ha-shatz: 1) the individual must not be proficient in prayer; one who knows how to pray must pray and beg for mercy on his own. She is unable to fulfill his obligation by hearing the ĥazan; 2) there must be a minyan, because the Sages instituted that individuals may only fulfill their obligation by hearing the ĥazan in the presence of a minyan; 3) the listener must understand the ĥazan’s words; one who does not understand Hebrew is unable to fulfill his obligation with Ĥazarat Ha-shatz.
The audience, even those who prayed individually, must take care to answer “amen” after every berakha of Ĥazarat Ha-shatz, as the Sages teach (Berakhot 53b): ‘One who responds “amen” is greater than the one who recites the blessing.’
One must respond “amen” with the utmost solemnity and have kavana that the content of the berakha is true. For instance, if one hears Ha-Kel Ha-Kadosh, she must have in mind: “It is true that the Lord is the holy God.” When the berakha includes a request, she should also think, “if only the request would be accepted.” For example, one who hears Ĥonen Ha-da’at should have two things in mind: 1) it is true that God grants knowledge; 2) “if only God would grant us knowledge” (SA 124:6; MB 25).
Here is the place to expand a bit on the laws of answering “amen” to the berakhot of Ĥazarat Ha-shatz and to all berakhot in general. “Amen” should be recited in a pleasant tone, and one’s voice should not be raised above that of the person reciting the berakha (SA 124:12). One must not shorten the “amen,” but extend it for the amount of time it takes to say “Kel Melekh Ne’eman,” although it should not be overly extended.
One may not recite “amen” too early, that is, before the berakha is completed; this is called an Amen Ĥatufa. Nor may one shorten the “amen” by slurring its syllables or tailing off before fully articulating it; this is called an Amen Ketufa. Nor may one delay saying “amen” for too long once the berakha has been concluded; this is called an Amen Yetoma (an orphaned Amen) (see Berakhot 47a; SA 124:8).
The way one responds “amen” corresponds to one’s faith in God. Because our lives depend on God, a defect in one’s faith produces a defect in his life. This is the meaning of Ben Azai’s statement: “Whoever responds with an Amen Yetoma – his children will be orphans; an Amen Ĥatufa – his days will be snatched from him; an Amen Ketufa – his days will be abbreviated. However, if one prolongs the recitation of “amen” – his days and years are lengthened” (Berakhot 47a).
The greatest Rishonim customarily said “barukh Hu u-varukh shemo” (“blessed is He, and blessed is His name”) when God’s name was mentioned in a berakha. This practice became widespread throughout Jewry, but it applies only berakhot that do not constitute the fulfillment of one’s personal obligation, such as Ĥazarat Ha-shatz for one who has already prayed silently. However, berakhot through which one fulfills a personal obligation, such as the berakhot on kiddush and blowing the shofar, the common practice is not to recite “barukh Hu u-varukh shemo,” so as not to interrupt the berakha with words not ordained by the Sages. Nevertheless, be-di’avad, if one responded “barukh Hu u-varukh shemo” in a berakha that she was obligated to recite, she has still fulfilled her obligation, since her response did not distract her from the berakha (MB 124: 21; Kaf Ha-ĥayim 26; some North African communities customarily respond even during berakhot through which one fulfills an obligation, whereas Yemenite custom is never to recite it at all).