The Kaf Ha'chayim (by Rabbi Yaakov Hayim Sofer) in siman 268, s"k 11-12, writes, based on the writings of the Arizal, that a person accepts the "Neshama Yeteira" – the "additional soul" that envelops a person on Shabbat – in three stages during the Friday night prayer service. The first component of the additional soul is referred to as "Nefesh," and it descends upon a person when he recites the words "Bo'i Kala" ("Come, O bride") at the end of the Lecha Dodi hymn. A person accepts the second level, that of "Ru'ach," when he responds to Barechu at the beginning of Arbit. For this reason, many people make a point of standing for the response to Barechu on Friday night. The final component, which is called "Neshama," is acquired during the recitation of the words "U'fros Aleinu…" just before the Amida prayer, such that a person accepts the Neshama Yeteira in its entirety by the time he recites the Amida on Friday night.
The Kaf Ha'chayim emphasizes that one must intend to accept each component of the Neshama Yeteira at each of the three points mentioned above. If a person does not have this intention, he cannot receive the special dimension of spirituality afforded by the Neshama Yeteira. The Kaf Ha'chayim notes that one who forgot to accept the component of "Ru'ach" when answering Barechu should have in mind when reciting the Barechu at the end of Arbit to accept simultaneously the components of "Ru'ach" and "Neshama." One cannot accept the level of "Neshama" when reciting "U'fros aleinu" if he had not intended to accept the element of "Ru'ach" during Barechu at the beginning of Arbit.
Summary: When one recites "Bo'i Kala" during the Friday night service, he should have in mind that he accepts the "Nefesh" component of the "additional soul" that envelops a Jew on Shabbat. When answering to Barechu, one should have in mind that he accepts the dimension of "Ru'ach," and when he recites, "U'fros Aleinu," he should think that he now accepts the final level of "Neshama." If one forgot to accept the level of "Ru'ach" during Barechu, he should accept simultaneously the levels of "Ru'ach" and "Neshama" during the final Barechu recited after Arbit.
