The following line is to be said silently:
Blessed is the name of the glory of His sovereignty for all time
As Rabbi Shimon ben Lakish said that it is written: “And Jacob called his sons and said, Gather around and I will tell you what will occur to you in the end of days” (Genesis 49:1). Jacob wanted to reveal to his sons when the complete redemption would arrive at the end of days (see Daniel 12:13), but the Divine Presence abandoned him, rendering him unable to prophesy. He said: Perhaps the Divine Presence has abandoned me because, Heaven forfend, one of my descendants is unfit, as was the case with my grandfather Abraham, from whom Ishmael emerged, and like my father Isaac, from whom Esau emerged. His sons said to him: Hear Israel, our father, the Lord is our God, the Lord is One. They said: Just as there is only one God in your heart, so too, there is only one in our hearts. At that moment Jacob our father said in praise: Blessed be the name of His glorious kingdom for ever and ever, as all his children were righteous. The Rabbis said: What should we do? Shall we recite this verse? But Moses our teacher did not say it in the Torah as part of Shema. Shall we not recite it? But Jacob said it. In order to resolve this dilemma they established that this passage should be recited surreptitiously. Rabbi Yitzḥak said that the school of Rabbi Ami said: This is analogous to the daughter of a king who smelled the fragrance of the dried spices stuck to the bottom of the pot and craved to eat them. What can she do? If she tells her servants to give it to her, she will be disgraced, as the dried spices are a contemptible food. However, if she does not say she wants to eat them, she will endure suffering. Her servants began to bring them to her surreptitiously.
Another idea: "Hear O Israel" The rabbis said: When Moshe went up to the heavens, he heard the ministering angels saying to the Holy Blessed One, "Blessed is the name of the glory of His sovereignty for all time" and brought it down to Israel. And why doesn't Israel say it in public (aloud)? Rabbi Asi said: To what is it similar? To one who stole jewelry from the King's palace and gave it to his wife and said to her, "Don't adorn yourself with it in public, rather (wear it) in your house." However, on Yom Kippur when they are as clean as the ministering angels, they say it publicly, "Blessed is the name of the glory of His sovereignty for all time."