Putting the Face Down [נפילת אפים]
Reason
We find a reason for Tachanun with the face down after Shemoneh Esrei. It shows that we davened in all sorts of manners to arouse Hashem’s mercy. We davened sitting until Shemoneh Esrei; we davened Shemoneh Esrei standing; then we say Tachanun with the face down. This is like what Moshe Rabbeinu did: “ואשב בהר” (דברים ט', ט'); “ואנכי עמדתי בהר” (דברים י', י'); “ואתנפל לפני ה'” (שם ט', י"ח ) (אבודרהם, טור, מ"ב סק"ט ).
ואנחנו לא נדע. The widespread minhag is to say “ואנחנו לא נדע מה נעשה כי עליך עינינו ” at the end of Tachanun (רמ"א ס"א ) to show that after we davened in all three manners – sitting, standing, and with the face down – we do not have the strength to daven in any other way; we beseeched Hashem in all the ways we could. Hence, “We do not know what we will do” (מ"ב שם סק"ט ). It is proper to say “ואנחנו לא נדע ” sitting, and “מה נעשה ” standing (של"ה ), because the act of standing when saying “מה נעשה ” hints at the fact that we davened sitting, standing, and then with our face down – what more can we do? “Our eyes are toward You...” (ערוך השלחן ס"ט ).
Intent of Putting the Face Down
After Shemoneh Esrei, we say Tachanun with the face down. Rabbeinu Bechaye provides multiple intents and purposes for this (פ' קרח ט"ז, כ"ב עה"פ 'ויפלו על פניהם ').
Awe of the Shechina
One reason is that one should have kavana when he davens that the Shechina is before him. Thus, it was established as part of the tactics of tefila that the face should be covered. This is for yiras Shomayim, as covering the face demonstrates humility and modesty.
Teshuva
Another reason is that it is a component of teshuva, showing pain and subservience. If one does this, his Shemoneh Esrei will be better received and Hashem will pay attention to his pain and fulfill his request.
Submission to Hashem
Another reason is to show submission of the emotions and shutting of the senses. By covering the eyes and closing the mouth, one shows he cannot accomplish his desires if Hashem does not consent.
Mesirus nefesh
Another idea mentioned in the Zohar is that there is a concept of mesirus nefesh in putting the face down. After one davened Shemoneh Esrei, he must show Hashem he is willing to give up his life for Hashem. One shows this by putting his face down, as if he ח"ו passed away, and by saying the paragraph from Tehillim “אליך ה' נפשי אשא” (תהילים כ"ה ).
Warning of the Zohar
The Zohar goes on to warn that according to the reason he gives, the paragraph must be said with the proper kavana and the feeling that one is truly prepared to give up his life for Hashem; then Hashem will have mercy on him and provide atonement. If one only says it outwardly but does not feel it in his heart, he endangers himself, as he is speaking falsehoods (זוה"ק במדבר ח"ג ק"כ, ב', הובא בב"י ד"ה כתוב בספר ובמג"א סק"ה ). It turns out that his display of mesirus nefesh is a worthless display; in fact, it is a mockery, and one can ח"ו get a serious punishment for this.
Changes due to the severity of the Zohar’s warning
Because of the severity of the Zohar’s warning about one who does not have proper kavana when putting the face down (above, 20), the minhag is to make some changes to how it is done in practice so that it is not done in the exact form the Zohar says. This way, one will not be caught ח"ו in the severe punishments described by the Zohar, as will be explained אי"ה.
