Before Birkhot Keri’at Shema, the ĥazan says, “Barkhu et Hashem ha-mevorakh” (“Bless God, the blessed One”), and the congregation responds, “Barukh Hashem ha-mevorakh le-olam va’ed” (“Blessed is God, the blessed one, forever and all time”), and the ĥazan repeats, “Barukh Hashem ha-mevorakh le-olam va’ed” (SA 57:1).
When the ĥazan says “Barkhu,” he bows a bit, and when saying “Hashem” he straightens up. Regarding the congregation, there are different customs. Some customarily bow down completely, others bow slightly, and still others do not bow at all. Everyone should follow family custom. When people with different customs pray together, it is proper for everyone to bow slightly (see Peninei Halakha: Prayer, ch. 17 n. 3).
The primary purpose of Barkhu is to introduce Birkhot Keri’at Shema, for by declaring “Barkhu,” the ĥazan invites the congregation to recite Birkhot Keri’at Shema. Likewise, those called up to the Torah recite Barkhu as an introduction to Birkhot Ha-Torah.
It is customary to repeat Barkhu at the end of the prayer service so that latecomers who missed the first Barkhu have the opportunity to respond to it (see Peninei Halakha: Prayer, 16:3 n. 2; 23:9).
There are different customs of standing when responding to Kaddish and Barkhu. According to the custom of most Sephardim, there is no need to stand while responding to devarim she-bikdusha, though but one who is already standing must remain standing for Kaddish and Barkhu (Maharil, Kaf Ha-ĥayim 56:20; 146:20-21; Yeĥaveh Da’at 3:4). Most Ashkenazim customarily stand while responding to Kaddish and Barkhu (MB 56:7-8; 146:18). However, concerning Barkhu, which requires a short answer, many Ashkenazim have a practice that if they are already sitting, such as during Torah reading or before Ma’ariv, they do not completely stand up. Rather, they rise slightly from their seats while bending their heads forward chairs, this rising and bowing a bit while answering. Many have a similar custom when answering a zimun of ten men, for although it is a davar she-bikdusha for which it is proper to stand according to Ashkenazic custom, since its recitation is short, it is sufficient to slightly rise instead of completely stand.