6:8 When we hear another person saying a bracha we should respond as follows. When he says “Baruch atah Hashem” (“Blessed are You, God”), we should respond “Baruch Hu u’baruch shmo” (“Blessed be He and blessed be His Name”). When the person finishes his blessing, we should say “Amen,” affirming our belief in the veracity of what the person just said (e.g., that God created the fruit that grows on trees, or that He sanctified us with such commandments as washing our hands, or whatever the bracha may be). When saying Amen, we should really think about the thing we are affirming as true. When saying Amen to a blessing that is also a prayer, such as those in Shemoneh Esrei, we should also have in mind a desire that it be God’s will to fulfill the prayer soon. When we say Amen to Kaddish, which speaks of future events, we should intend that God accept the words of the prayer.
6:9 If a person hears a bracha but he is praying and is at a place where one may not interrupt, then he does not say “Baruch Hu u’baruch shmo.” Also, if another is saying a blessing through which we will fulfill our own obligation, such as when one person recites Kiddush for others, we do not say “Baruch Hu u’baruch shmo.” This is true before hearing the shofar on Rosh Hashana and the megilla on Purim. Answering “Baruch Hu u’baruch shmo” constitutes an interruption when one is hearing a blessing meant to discharge his own obligation. The Mishnah Brurah (124:21) says that if one did say “Baruch Hu u’baruch shmo” in such a case, even though he shouldn’t have interrupted, the bracha recited still covers him. (The specifics about answering “Amen” while in the middle of davening are discussed later on, in chapters 10, 14 and 16.)