The unification of the Shema Israel verse was made by Moses with 25 letters. These letters are carved from a supernal secret of Zeir Anpin. The 25,כ”ה, implies to the 22 letters of the Torah and the three parts of the Bible (Torah, Prophets, Writings) that are the secret of the three lines of Right, Left, and Center.
Jacob Established the unification in Malchut with ‘Baruch Shem’ that has 24 letters. He didn’t use 25 letters because the Holy Tabernacle that is the connection of the Light to Malchut was not established in his time.
After the building of the Holy Tabernacle that enabled the unification of Zeir Anpin and Malchut, the connection could be made with 25 letters. We see it when YHVH called Moses for the first time from the Holy Tabernacle using 25 letters.
Leviticus 1: 1
“וַיְדַבֵּר יְהוָה אֵלָיו, מֵאֹהֶל מוֹעֵד לֵאמֹר” (count of 25 letters)
“and spoke unto him out of the tent of meeting, saying”
There were 25 different elements that were needed to complete the Holy Temple; 13 types of Gold, Silver, Copper, etc. and the 12 stones on breast place of the High Priest. For that reason, the holy aspect of the Temple is called כה (numerically 25). It represents the complete unification of Zeir Anpin and Malchut in the Temple.
“יוֹדוּךָ יְהוָה כָּל מַעֲשֶׂיךָ וַחֲסִידֶיךָ יְבָרֲכוּכָה.”
“All Your works shall give thanks to You, O YHVH, And Your godly ones shall bless You כה.”
‘shall bless You’ is ‘יְבָרֲכוּכָה’, a word that we see as two יְבָרֲכוּ כָה. We bless ‘כה’ that is the secret of holiness that comes to Malchut.
The origin of “Baruch Shem” as described in the Talmud (Pesachim 26)
When Jacob was close to his death, he called his sons and wanted to tell them the future but the Shechina left him. He asked them if any of them is in the impure state (he assumed that the Shechina left him because of them) and they all answered together “Shema Israel…”
“שְׁמַע יִשְׂרָאֵל יְהוָה אֱלֹהֵינוּ יְהוָה אֶחָד.”
“Hear, O Israel! YHVH is our God, YHVH is one!”
Israel was the name given to Jacob by the angel and his children used it to tell their father ‘Hear O Israel’, meaning listen to us. Then they made the unification of the three names YHVH saying יְהוָה אֱלֹהֵינוּ, יְהוָה, YHVH, ELOHENU, YHVH is One!
He replied to them “בָּרוּךְ שֵׁם כְּבוֹד מַלְכוּתוֹ לְעוֹלָם וָעֶד” “Blessed be the name of the glory of his kingdom forever and ever”
The sages also attribute the ‘Baruch Shem’ to Moses that heard it from the angels when he went up the mountain to receive the Torah. Because Moses ‘copied’ from the angels, we whisper this verse because we are lower than them. On Yom Kippur, we are on the higher level of Binah like the angels and for that reason, we say the Baruch Shem aloud.
We say the Baruch Shem after the Ana B’Choach because it is a unification between Malchut and a secret of the 42 letter name.
Numbers 6:23 “דַּבֵּר אֶל-אַהֲרֹן וְאֶל-בָּנָיו לֵאמֹר, כֹּה תְבָרְכוּ אֶת-בְּנֵי יִשְׂרָאֵל: אָמוֹר, לָהֶם”
“Speak to Aaron and his sons, saying, Thus (כֹּה) you shall bless the people of Israel: you shall say to them,”
The priests bless us with the word כה that represents the Shechina in Malchut that is the source of blessings for us.