Should Yishtabah be recited during Barechu?
The Shulhan Aruch (O.H. 57:1) records a practice in which the congregation says several verses, beginning with the word “Yishtabah”, while the Shaliah Tzibur stretches out the recital of “Barechu,” such as on Shabbat or holidays. This is a prevalent custom in the Moroccan community and the full text of can be found in all Moroccan Siddurim next to Barechu. The Rosh (§ 4:19), the Tur (O.H. § 57) and the Avudraham all write that this custom existed and was prevalent in Spain in the era of the early Rishonim. Furthermore, the Arizal (Sha’ar Hakavanot, pg. 51) encourages the recital of Yishtabah when Barechu is being said.
The Magen Avraham (M.A., O.H. 57:3), posits that one should listen to Barechu being recited by the Shaliah Tzibur and therefore one may only say the words of Yishtabah when the words of Barechu are being stretched in singing. Rabbi Baruch Toledano, contrary to the mainstream Moroccan practice, concurs with the Magen Avraham. The aforementioned Rishonim who cite this custom do not make a distinction as to when Yishtabah should be said, and as such, the prevalent Moroccan custom is to recite Yishtbah even when the Shaliah Tzibur is saying the words of Barechu. Additionally, Rabbi Ya’akov Hillel (Mekabtziel, vol.XXXI, pg.121) says that from the language of the Arizal, he seems to imply that Yishtbah should be said specifically when the Shaliah Tzibur is saying the words of Barechu.
This is similar to the custom of reciting Kedusha in sync with the Shaliah Tzibur, even though one may have thought that one should be silent and listen to him recite it. One way to reconcile this is that one gives validity to the a blessing or prayer when one listens silently and responds to it. So too in this case, by reciting the verses one is actively aware of what the Shaliah Tzibur is saying even though one is not silently listening and, indeed, these verses are a validation of what the Shaliah Tzibur is saying. Furthermore, the Gemara (Rosh Hashana 27a) says that although two simultaneous voices are incomprehensible, when whatever is being listened to is dear to someone, there is no deficiency in making out those two voices.
This custom should be strengthened as it has bona fide sources and because its words praise Hashem and give great honor to the recital of Barechu and to the subsequent blessings of Shema. Although Yishtabah could be recited anytime Barechu is said, it is typically done one Shabbat and holidays when there is a tendency for the Shaliah Tzibur to chant, thus giving the congregants ample time to say all the verses.
Summary: The Moroccan custom is to recite the verses of Yishtabah while the Shaliah Tzibur recites Barechu.