The following Halachot were taught by Rav Gideon Ben Moshe regarding interrupting Tefila during Pesukeh D’zimra, from after Baruch She’amar until the end of Az Yashir. It is permitted to answer Barchu and at least the first five Amens of a Kadish, until "Da’amiran B’alma." It is also permitted to answer Amen to someone’s Berachot, such as the Beracha on Talit or Tefilin, or the Hazan’s Beracha at the end of Baruch She’amar.
The Halacha becomes more complex regarding answering while in the middle of Baruch She’amar. During the first half, there is no problem whatsoever, as it is merely verse of praise without a Beracha. There, one can answer anything- even the last Amens of Kadish, or "Baruch Hu U’varuch Shemo," which cannot be answered during the rest of Pesukeh D’zimra. However, during the second half of Baruch She’amar, which is actually one long Beracha, the Halacha is strict and prohibits interrupting to answer Amen to Berachot. For example, if the Hazan reaches the end before one completes the Beracha, he should not answer Amen to the Hazan’s Beracha. One can only answer Devarim She’bikdusha-such as Barchu and Kedusha, which require a Minyan to recite. These Halachot are commonly known and are recorded in the charts at the back of the Siddurim.
However, Rav Gideon Ben Moshe added that the conventional understanding regarding interrupting during Yishtabach is flawed. Yishtabach is not an independent Beracha; it is a "Beracha Semucha L’haverta"-adjoined to Baruch She’amar. Therefore, the entire Yishtabach is one long Beracha from beginning to end, and it has the status of the second half of Baruch She’amar, and Amen may not be answered. According to Rav Ben Moshe, some Siddurim and the Halacha Berura, mistakenly understood that one can answer Amen during Yishtabach, based on a misreading of the Ben Ish Hai who compared Yishtabach to Baruch She’amar.
