The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) cites (122:8) the work Seder Ha’yom which emphasizes the value and significance of reciting the verse "Yiheyu Le’rason Imreh Fi Ve’hegyon Libi Lefanecha Hashem Suri Ve’goali" at the end of the Amida. The Seder Ha’yom writes that this verse contains several "Segulot" and is laden with deep significance, and he adds, "Ve’sodo So Gadol" – great secrets are embedded within it. The Mishna Berura writes that this verse must be recited slowly and with concentration, and that this recitation is exceedingly beneficial in ensuring the acceptance of one’s prayers.
For this reason, one may not make any verbal interruption, such as answering to Kaddish or Kedusha, or answering "Amen," until he recites "Yiheyu Le’rason." Even if somebody recited "Oseh Shalom," he may not make any interruption if he had not recited the verse of "Yiheyu Le’rason." It must be noted that the Hazan, too, must recite "Yiheyu Le’rason" after the repetition of the Amida. After reciting "Ha’mebarech Et Amo Yisrael Ba’shalom Amen," the Hazan should not immediately proceed to "Ana" or "Yehi Shem"; he must first recite "Yiheyu Le’rason."
One of the "secrets" to which the Mishna Berura refers is the observation made by the Arizal (Rav Yishak Luria of Safed, 1534-1572) that the letter "Yod" appears nine times in this Pasuk. These nine instances of the letter "Yod," the Arizal explained, correspond to the nine "Yods" that appear in the four different configurations of the divine Name of "Havaya" ("Shem Ab," "Shem Sag," "Shem Ma," and "Shem Ben"; listen to audio recording for an explanation of these different configurations). When we recite the verse "Yehi Le’rason" with this intention in mind, we activate these four Names, and the Sha’ar Ha’kavanot writes that this is the key to having our prayers accepted. Recognizing this special power of "Yehi Le’rason" should motivate us to pay particular attention to this verse and recite it slowly and with concentration.
Summary: The recitation of "Yehi Le’rason" at the end of the Amida is very significant, and has the power to ensure the acceptance of our prayers. Therefore, it must be recited slowly and with concentration. One may not speak or even answer to Kaddish or Kedusha after completing the Amida before reciting this verse. The Hazan must also recite this verse before proceeding to "Ana" or "Yehi Shem."