Background
The tefila of Tachanun said with the face down is a plea said throughout Klal Yisroel every day at Shacharis and Mincha after Chazaras HaShatz. Because we are so used to it, people do not realize its importance and nature or the uniqueness of its content. Also, since there are times it is not said, many people are lax about saying it and extend these times to other times. Thus, we saw a need to explain its basis and importance in order to rekindle the desire to say it with kavana and patience, the way it is meant to be said; and so that we will know when, in fact, it is not said according to halacha or an accepted minhag. Then, we will also realize the greatness of the days or events when we do not say it.
Meaning of “Tachanun.” Tachanun is a form of the word תחנונים [pleas]. In Chazal’s time, people added various personal requests and pleas for things they needed very much, each person in his own words (כל בו סי' י"א, ארחות חיים ), which flowed from the depths of the heart. This was an extension of the feelings of closeness to Hashem that they felt during Shemoneh Esrei. It is characterized as a request for a free gift, as in the posuk “וחנותי את אשר אחון” (שמות ל"ג, י"ט ) – I act [as with] a free gift (דברים רבה ב', א'). The nusach of the tefila of Tachanun we say today was established as a fixed nusach; it is cited in Seder Rav Amram Gaon.
Elements of Tachanun
Tachanun consists of multiple elements: Vidui, the 13 Middos Rachamim, putting the face down, additional tefilos, and והוא רחום on Monday and Thursday. We will explain each of them.
Vidui
Based on the Zohar (פ' פקודי דף רס"ב ע"א ), we say Vidui right after Shemoneh Esrei, before putting the face down. The idea is to confess before we as if “give up our lives,” which is the concept behind putting the face down according to the Zohar (below, 19) (ליקוטי מהרי"ח, שו"ת דברי יציב או"ח סי' ע"ד סוף אות ה'). This is the minhag of Nusach Sefard and Eidot Mizrach. Some Ashkenazim say Vidui on Monday and Thursday (טור קלד בשם רב עמרם, ומנהג יר ושלים ), but most who daven Nusach Ashkenaz do not say Vidui (טור שם ).
13 Middos
Since we say Vidui, we also say the 13 Middos shel Rachamim, which Moshe Rabbeinu said to ask for mercy and atonement from Hashem for Klal Yisroel after the Cheit HaEigel, as the posuk says (שמות ל"ג, י"ט ), “וקראתי בשם ה' לפניך, וחנותי את אשר אחון ורחמתי את אשר ארחם .” The Gemara (ר"ה יז :) says Hashem wrapped Himself like a shliach tzibbur and showed Moshe this process of tefila, saying “Whenever Klal Yisroel sins, they should perform before Me this process [the 13 Middos] and I will forgive them.”
Falling on the Face
In the time of Chazal, they actually prostrated themselves with their faces on the ground without extending their hands and feet [להבדיל the way the Arabs pray]. However, due to concern for improper bowing [see Shulchan Aruch (סי' קל"א ס"ח )], today we do not actually prostrate ourselves. In place of prostrating ourselves, we say Tachanun sitting and leaning on the arm with the face covered (מ"ב סק"ג ).
והוא רחום on Monday and Thursday
On Monday and Thursday, we say additional tefilos, i.e., והוא רחום וכו' (שו"ע ורמ"א סי' קל"ד ס"א).
Tefilos after putting the face down
We also say tefilos after putting our faces down, i.e., שומר ישראל (סידור השל"ה, ליקוטי מהרי"ח ) and ואנחנו לא נדע (רמ"א ).
Reshus or Chiyuv?
Chiyuv
Some say there is a chiyuv to say Tachanun, as it is a part of tefila, albeit one that is not critical (רמב"ם פ"ה תפילה ה"א ).
Reshus
Others hold one does not have a chiyuv to say Tachanun (שו"ת הריב"ש סי' תי"ב, טור סי' קל"א בשם רב נטרונאי גאון, דר כ"מ שם ). Yet even though it is not a real chiyuv, one should not skip it for no reason, as, at the very least, there is a real minhag to say it; only since it is not a chiyuv, wherever there is a minhag one should follow the minhag (רא"ה ברכות דף ל"א ע"א ).
Accepted as a chiyuv
Although we primarily pasken it is not a chiyuv, and therefore there are cases where people are meikel, since today all of Klal Yisroel says Tachanun, we have accepted it upon ourselves as a chiyuv to some extent since it is a very important matter (ערוך השלחן סי' קל"א ס"ב ).
Also, it is like a chiyuv based on Kabbala since it contains great hidden significance (below, 19). The same is stated in Shaar HaKavanos by the Arizal (דרוש ה' בחזרת העמידה, כף החיים אות נ"ג ). On days when Tachanun is not said according to halacha, the kedusha of the day accomplishes what the tefila would accomplish (שם אות נ"ד ).
