The Pesukeh De’zimra section of our morning prayer service concludes with the blessing of Yishtabah. The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) describes this Beracha (in Parashat Vayigash) as "Nora Ve’asum Me’od" – exceedingly significant and powerful. According to some opinions, this blessing was authored by King Shlomo and Abraham Abinu. The first letters of the words at the beginning of this Beracha, "Shimcha La’ad Malkenu Ha’Kel" spell the name "Shelomo," and the first letters of the words towards the end, "Kel…Boreh…Ribon…Ha’boher…Melech" spell "Abraham."
The Aruch Ha’shulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908) observes that the Shulhan Aruch begins his presentation of the laws relevant to the Hazan in Siman 53, only after he discusses the recitation of Pesukeh De’zimra. Apparently, the Aruch Ha’shulhan writes, long ago the Hazzan began with Yishtabah. The congregation recited the first part of the prayer service, until Yishtabah, without a Hazzan, and it was only at Yishtabah that the Hazzan began leading the service.
This Beracha speaks of 13 different kinds of praises which we ought to be expressing to Hashem ("Shir U’shabaha Hallel Ve’zimra…"), corresponding to the 13 Middot Rahamim (attributes of justice). These attributes, which we recite in full after the Amida prayer, are alluded to also in Baruch She’amar, the introductory Beracha to Pesukeh De’zimra, which contains 13 instances of the word "Baruch." The Ben Ish Hai writes that these 13 words are very significant, and so they must be recited slowly and in a pleasant melody. The Magen Abraham (Rav Abraham Gombiner, 1633-1683) writes that it is proper to recite these 13 words all in a single breath, without any pause or interruption, because they correspond to the single unit of the 13 attributes. The Ben Ish Hai, however, maintained that this should not be done, as one should recite these 13 expressions slowly, without rushing, albeit without any long pause or interruption. Hacham Yaakob Kasin (1900-1994), in Kesineh Erez, addresses this issue and cites the Ben Ish Hai’s ruling that one should not read these 13 words in a single breath, and should instead recite them slowly, with concentration.
If one hears Kaddish while reciting these 13 words, he should answer to Kaddish. According to the Ben Ish Hai, one must then return to "Shir U’shbaha" so he can recite the 13 expressions without any interruption. Hacham David Yosef, however, in Halacha Berura, writes that this is not necessary, and one may simply continue as usual after answering Kaddish.
One must ensure not to make any interruption from Baruch She’amar until the conclusion of Yishatabah. Therefore, if a person reaches Yishtabah before the Hazzan, and he wants to learn while he waits for the Hazzan, he should first recite Yishtabah. This is especially common on Shabbat, when Hazzanim often take some time before reaching Yishtabah. One who wishes to learn while he waits, even silently, should first recite Yishtabah, as not even silent interruptions are allowed in between Baruch She’amar and Yishtabah.
Summary: The 13 expressions of praise in Yishtabah ("Shir U’shbaha…U’malchut") should be recited slowly and with concentration. Although some maintained that they should be recited in one breath, it is preferable to recite them slowly. If one hears Kaddish while reciting these expressions, he should respond to Kaddish. Some Poskim rule that he should then return to "Shir U’shbaha," whereas others maintain that he may simply continue. One may not make any interruption – even a silent interruption – from Baruch She’amar through the end of Yishtabah. Therefore, if a person reaches Yishtabah before the Hazzan, and he wants to learn while he waits for the Hazzan, he should first recite Yishtabah.