Finishing the blessing of Habocher Beamo Yisrael Behava quietly:
 Sefer Haminhagim p. 25 [English]
Ruling of Admur in Shulchan Aruch: In the Shulchan Aruch 59/4 and 61/3 Admur rules that the Chazan is to recite the blessing aloud and one is to complete the blessing prior to the Chazan in order so they can answer Amen to it.
Other customs: Some are accustomed to conclude the blessing together with the Chazan in order to avoid the need to answer Amen. [Custom of Sefaradim and majority of Ashkenazi communities, brought in Beis Yosef 59 and so concludes Shelah; Shaareiy Teshuvah 59/5 and M”B 59/25 that so should be done initially despite the ruling of the Rama and Ashkenazi custom] Others are accustomed to complete the blessing prior to the Chazan in order so they can answer Amen to it. [Admur 59/4 and 61/3; Rama 59/4 and 61/3; Shulchan Hatahor 60/2 that so applies according to the Arizal; Mishmeres Shalom Minhagim 11]
 See Ketzos Hashulchan 19/1
 The reason: As it is disputed amongst Poskim as to whether one who is prior to Shema is to answer Amen to the blessing of Ahavas Olam. [See Admur 59/4] Therefore, in order to avoid the dispute, the Chazan concludes this blessing in silent.
 The Rebbe’s custom was to say the words Baruch Ata and part of the word Hashem aloud and then say the remaining words in silent. [heard in videos of the Rebbe Davening Shacharis as Chazan]