Our tradition teaches that we receive a "Neshama Yetera" ("extra soul") on Shabbat. We receive this soul on Friday night in three distinct stages, and we then receive a higher-level soul on Shabbat morning, also in three stages. On Friday night, we receive the "Nefesh" component of the extra soul when we recite "Lecha Dodi," specifically, when we recite, "Bo’i Kalla." The second component, "Ru’ah," is received at the time of "Barechu," and we receive the third component – "Neshama" – when we recite in "Hashkibenu" the words, "U’fros Alenu." On Shabbat morning, we receive the "Nefesh" aspect when we recite the "Nishmat" prayer; "Ru’ah" when we begin the Amida and recite "Ado-nai Sefatai Tiftah"; and the "Neshama" component when we recite "Ayeh" in the Kedusha of Musaf.
The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) writes that there are several Kavanot ("intentions") that even ordinary laymen should try to have during the recitation of "Nishmat." First, one should have in mind that the first letters of the opening words of this prayer, "Nishmat Kol Hai," are "Nun," "Kaf" and "Het," which have a combined numerical value 78, which is equivalent to three times the divine Name of "Havaya." Secondly, the words "Kol Hai" have a combined numerical value of 68 (the numerical value of "Haim"), which is associated with a divine Name that represents the Misva of Tefillin. There is no Misva to wear Tefillin on Shabbat, but by reciting "Nishmat" and having in mind the number 68 when saying the words "Kol Hai," one is considered to have worn Tefillin. In fact, Rav David Pardo (1718-1790) wrote that if one does not recite "Nishmat" on Shabbat morning, then when he later reads Shema he is considered as though he has recited Shema without Tefillin.
The third intention one should have when reciting "Nishmat," the Ben Ish Hai writes, is that through this recitation he brings the sanctity of Shabbat to the "Olam Ha’yesira."
The Ben Ish Hai adds that reciting "Nishmat" is so significant that one who forgot to recite "Nishmat" before reciting "Yishtabah" must recite it after "Yishtabah." If he realized his mistake only after he began "Yoser Or," then he should recite "Nishmat" after the Amida. In such a case, the person must think in his mind the first eight words of "Nishmat" before beginning the Amida, so he can receive the "Nefesh" before receiving the "Ru’ah" when he begins the Amida.
There are variant versions of the text of "Nishmat." The Ben Ish Hai was very adamant that the phrase, "Yodu Vi’varechu Vi’shabehu Vi’fa’aru" should be followed by the word "Vi’shoreru," as these five terms correspond to the five components of the soul. By contrast, the Kaf Ha’haim (Rav Yaakob Haim Sofer, Baghdad-Israel, 1867-1939) cites an opinion that the word "Vi’shoreru" should be omitted, and some Siddurim follow this view.
Toward the end of the "Nishmat" prayer, we say that Hashem hears "Shav’at Aniyim" – "the plea of the poverty-stricken" – as well as "Sa’akat Ha’dal" – "the cry of the destitute." The term "Aniyim" refers to the Jewish People in exile, whereas "Ha’dal" refers to Mashiah Ben Yosef, who suffered as a result of our sins. When G-d takes us out of exile, He will answer our prayers as well as that of Mashiah Ben Yosef, who will be taken out of his state of suffering and will be redeemed along with the rest of the Jewish Nation.