During the Asseret Yemeh Teshuba, the ten days between Rosh Hashana and Yom Kippur, it is customary recite chapter 130 from the Tehilim, Shir HaMa’alot Mi’mamakim, in between Yishtabach and Kadish during Tefilat Shacharit. The source of this practice is the Sha’ar Hakavanot of Rabbenu HaAri (Rav Yishak Luria of Safed, 1534-1572). Therefore, it has great significance on a mystical level to mitigate and sweeten harsh judgments.
The Magen Avraham raises the question why inserting this extraneous chapters does not constitute a Hefsek (interruption of prayer). Hacham Ovadia (Hazon Ovadia, Yamim Noraim p.189) answers that it is not considered a Hefsek since it is a necessary addition. As a proof, he cites a precedent in which the Shulhan Aruch (Siman 54) permits making an important appeal for charity between Yishtabach and Kadish.
However, if they forgot to say the chapter until after Kadish was already recited, it is no longer possible to do so, since Barchu must immediately follow the Kadish, and after Barchu, no interruptions are allowed.
SUMMARY
It is an important custom to recite Chapter 130 of Tehilim after Yishtabach during the Asseret Yemeh Teshuba. If they only remembered to do so after Kadish, it is not recited.
