The Midrash Rabbah on Parashas Vayechi brings a striking teaching on the verse (בראשית מט יח): לִישׁוּעָתְךָ קִוִּיתִי ה'. אָמַר רַבִּי יִצְחָק הַכֹּל בְּקִוּוּי - For Your salvation I have hoped, Hashem. Reb Yitzchak says: "Everything depends on hope - kivui."
This reveals the extraordinary power that kivui carries in even the most desperate of straits. One who truly believes in Hashem - even if he has descended into Gehinnom itself - will be lifted out of it.
The Ramchal derives this from the verse (ישעיה מ לא): וְקוֹיֵ ה' יַחֲלִיפוּ כֹחַ יַעֲלוּ אֵבֶר כַּנְּשָׁרִים - Those who hope to Hashem will renew their strength; they will soar on wings like eagles. Through his emunah, one is raised up from the pit - and angels on high come to lift him, ascending together with him to a place beyond judgment. Then, the Ramchal adds a teaching of stunning beauty: "His hope is his purification - mikveh Yisrael mamash." The Navi says (ירמיה יד ח): מִקְוֵה יִשְׂרָאֵל ה' מוֹשִׁיעוֹ בְּעֵת צָרָה - Hashem is the hope (מקוה) of Yisrael, Deliverer in time of trouble. The word mikveh means both hope and a pool of purifying waters. The one who hopes in Hashem stands at such an elevated level that sin cannot take hold of him - his kivui itself is his taharah. (רמח"ל, דרוש הקיווי)
The Yesod V'Shoresh HaAvodah (פרק ו) writes that when a Yid recites the words "וְעַל כֵּן נְקַוֶּה לְּךָ ה' אֱלֹקֵינוּ" in Aleinu, he should fill his heart with an immense and trembling joy - "as if he is hearing, at that very moment, Eliyahu announcing that today the Shechinah is leaving exile." That is what kivui looks like at its fullest.
The tzaddikim each had their own path in expressing the kivui for Moshiach: The holy Rebbe of Belz, R' Aharon zt"l - and it is similarly told of the Kedushas Levi zt"l - would not store away his Kinnos from one year to the next. His emunah in the coming geulah was so vivid and present that he could not imagine needing them again.
The holy Vayaggeid Yaakov of Papa zt"l would place his Kinnos in genizah entirely each year after Tisha B'Av, consigning them to rest - as one who has no further use for a document of mourning.
But his son, the Vaychi Yosef of Papa zt"l, charted a third path - he kept the Kinnos from the previous year, and yet each year he went and purchased a new one in addition to the old one. He explained his reasoning simply: at the time of the future geulah, every lament will be transformed into a song of joy and celebration. And so, he was preparing in advance - accumulating Kinnos, year after year - so that when Moshiach Tzidkeinu arrived, he would have enough to distribute to many, many people, that they might all sing together.
