When Rosh Hashana falls on Shabbos, the entire congregation repeats וַיְכֻלּוּ, in testimony of our belief that God created Heaven and earth. According to the Halacha, when one is a witness, he must stand and declare his testimony aloud.
וַיְכֻלּוּ הַשָּׁמַֽיִם וְהָאָֽרֶץ וְכָל־צְבָאָם: וַיְכַל אֱלֹהִים בַּיּוֹם הַשְּׁבִיעִי מְלַאכְתּוֹ אֲשֶׁר עָשָׂה. וַיִּשְׁבֹּת בַּיּוֹם הַשְּׁבִיעִי מִכָּל־מְלַאכְתּוֹ אֲשֶׁר עָשָׂה: וַיְבָֽרֶךְ אֱלֹהִים אֶת־יוֹם הַשְּׁבִיעִי וַיְקַדֵּשׁ אֹתוֹ. כִּי בוֹ שָׁבַת מִכָּל־מְלַאכְתּוֹ אֲשֶׁר־בָּרָא אֱלֹהִים לַעֲשׂוֹת: The heavens and the earth were completed and [so were] all their hosts. And God completed by the seventh day His work which He had done, and He abstained on the seventh day, from all His work which He had done. And God blessed the seventh day and sanctified it for on it He abstained from all His work which God had created to do.
The following prayer, בְּרָכָה מֵעֵין שֶׁבַע the abridged form of the seven blessings of the Maariv Amidah, is said only by the Chazzan.
חזן: בָּרוּךְ אַתָּה יְהֹוָה אֱלֹהֵֽינוּ וֵאלֹהֵי אֲבוֹתֵֽינוּ אֱלֹהֵי אַבְרָהָם אֱלֹהֵי יִצְחָק וֵאלֹהֵי יַעֲקֹב הָאֵל הַגָּדוֹל הַגִּבּוֹר וְהַנּוֹרָא. אֵל עֶלְיוֹן קוֹנֵה שָׁמַֽיִם וָאָֽרֶץ: Blessed are You, Adonoy, our God and God of our fathers, God of Avraham, God of Yitzchak, and God of Yaakov; the Almighty, the Great, the Powerful, the Awesome, the most high Almighty, Possessor of heaven and earth.
The congregation may recite the following paragraph, which should then be repeated aloud and in its entirety by the Chazzan.