It is forbidden to interrupt one’s recitation of the Amida at any point, even to answer to Nakdishach (Kedusha), until the first recitation of the verse, "Yiheyu Le’rason Imreh Fi."
What, then, should one do if he is still reciting the Amida when the Hazan reaches Nakdishach? Should he simply continue reciting the Amida, since in any event he cannot answer to Nakdishach, or is it proper for him to remain silent during the congregation’s recitation of Nakdishach?
Hacham Ovadia Yosef addresses this question in his work Halichot Olam, and rules that the Halacha in this case depends on one’s assessment of the Hazan. If the Hazan recites Nakdishach loudly and clearly, such that he can be heard by the congregation, and it can be assumed that he has in mind to fulfill the obligation of those listening to his recitation, then one should, indeed, pause during Nakdishach. Instead of continuing his Amida, he should remain silent and listen attentively to the Hazan’s recitation of Nakdishach, so that he can be considered to have recited it himself. It should be noted that, as Rav Haim Palachi (Turkey, 1788-1869) writes in his work Kaf Ha’haim in the name of the Hesed La’alafim, a person in this case should not lift his feet off the ground during the Hazan’s recitation of "Kadosh, Kadosh." Since he does not personally recite "Kadosh, Kadosh," there is no reason for him to lift his feet. Moreover, if he lifts his feet, others may mistakenly think that he is actually reciting Nakdishach, and they will thus reach the erroneous conclusion that one may interrupt the Amida to recite Nakdishach. Therefore, in a case where one remains silent to listen to Nakdishach during his Amida, he should not lift his feet for the recitation of "Kadosh, Kadosh."
However, if the Hazan recites Nakdishach inaudibly, or if he likely does not realize that his recitation fulfills the obligation for those listening, then there is no purpose to pausing during one’s Amida for Nakdishach. Since he does not hear the Hazan’s recitation, and the Hazan most likely does not intend for his recitation to fulfill the obligation of others in the synagogue, nothing is gained by pausing. In such a case, then, one should simply continue reciting his Amida, even as the Hazan and congregation recite Nakdishach.
This Halacha should serve as a reminder for Hazanim to ensure to recite Nakdishach – particularly the passages, "Kadosh, Kadosh" and "Baruch Shem" – loudly and clearly, such that their voices can be heard over the congregation. They must also have in mind that those in the congregation who are still reciting the Amida can fulfill the Misva of Nakdishach by listening to the Hazan.
Summary: A person may not interrupt his Amida to respond to Nakdishach. However, if the Hazan recites Nakdishach loudly, and can be presumed to know the Halacha requiring that he have in mind to fulfill the obligation for those listening, then the person should pause and listen silently to the Hazan’s recitation of Nakdishach. Otherwise, he should simply continue the Amida, even during Nakdishach.
