Pesukei d’Zimra (lit. “Verses of Song”) is the section of Shacharis that is filled with praises of Hashem, taken mostly from Tehillim (Book of Psalms). The Sages instituted that this section should begin and end with a bracha. These brachos are known as ‘Baruch She’Amar’ (“Blessed is He Who spoke”) and ‘Yishtabach’ [“May (Your Name) be praised”], respectively. (Ed: This entire section is found in the Artscroll Siddur pp. 58-82.)
The origin of ‘Baruch She’Amar’ appears to be quite mysterious. It is not mentioned in the Gemara, which is the source of most of halacha; rather, it was said to have “fallen from Heaven”, written on a piece of parchment! It contains 87 words. This number corresponds to the numerical value of the Hebrew word ‘Paz’ (gold), alluding to the phrase, “At its head (i.e. the “head” of the ‘Pesukei d’Zimra’), a golden form.” Actually, the first half-or-so of ‘Baruch She’Amar’ consists of “loose” praises, and the real bracha begins with ‘Baruch Atah Hashem… ha(K)eil haAv haRachaman…’ (“Blessed… the G-d, the merciful Father…”). (Note: Knowing the actual starting point of the bracha is relevant to the matter of interruptions, as will be discussed ahead.) It is proper to stand during ‘Baruch She’Amar’. It is also customary for a boy or man to hold his two front ‘tzitziyos’ (i.e. the fringes on his talis or talis-katan) during the recitation of ‘Baruch She’Amar’ and to kiss them at its end.
Question: Is one permitted to answer ‘amein’ to brachos that one hears, or make other responses, once he has begun the ‘Pesukei d’Zimra?
Answer: Generally-speaking, mitzvah-responses such as ‘amein’ may be made during ‘Pesukei D’Zimra’. This includes even while someone is in the middle of the bracha-part of ‘Baruch She’Amar’ itself* (Note: except during its final conclusion, i.e. ‘Baruch… Melech mehulal batishbachos’ – “Blessed… the King Who is lauded with praises”). (Ed: More on other responses – both mitzvah- and non-mitzvah-related – later.)
(*The only exception to this is one’s answering ‘amein’ to someone else’s ‘Baruch She’Amar’, while he is in the middle of it himself!)
For now, here is an interesting “side-bar” which the Mishnah Berura brings up here:
May one answer ‘amein’ to his own bracha?!
No, one may not answer ‘amein’ to his own bracha, unless:
1) He is finishing his bracha at precisely the same time that someone else is finishing a different bracha (in which case, he is actually saying ‘amein’ to the other person’s bracha, not his own.) OR
2) He is finishing his bracha at precisely the same time that someone else is finishing the same bracha, which happens to be a bracha that concludes an entire series of brachos. For example, if one is finishing ‘Baruch She’Amar’ at the same time that the chazzan is finishing it, he may not answer ‘amein’. If he is finishing ‘Yishtabach’ at the same time that the Chazzan is finishing it, he may answer ‘amein’. Why the difference? Because the former is not the end of a series; the latter is. Other examples of “ends of series’” are the bracha at the conclusion of ‘Hallel’ (p. 642) and the second bracha after ‘Krias Shema’ of Maariv (‘Baruch…Shomer Amo Yisrael La-ad’ – p. 264.)