The Tur (Orach Chaim 131) writes, "We daven in every position: sitting, standing, and fallen on the face. Moshe Rabbeinu did that, as it states (Devarim 9:9), בהר ואשב, 'I sat on the mountain,' and it states (ibid. 10:10), עמדתי ואנכי בהר, 'I stood on the mountain,' and it states (ibid. 9:25), 'ה לפני ואתנפל, 'I fell before Hashem.'"
The Bach explains that first, we daven sitting down [when we say Shema with its brachos], then we stand up for Shemonah Esrei, and then we fall on our faces for tachanun, because we want to daven in these three ways. Afterward, we say, מה נדע לא ואנחנו נעשה, which means we don’t know of any other form of tefillah. If we did, we would daven in those ways, too, as we seek to pray and beseech Hashem in every possible way. Maybe we will find a way to arouse Hashem's compassion and answer our tefillos.
In this week's parashah, Moshe and Aharon fell on their faces and prayed that Hashem not destroy the Jewish nation (see 16:22), כל ועל יחטא אחד האיש ...ויאמרו פניהם על ויפלו יקצוף העדה, "Moshe and Aharon fell on their faces and said... 'If one person sins, will You be angry at the entire community?'" Hashem accepted their tefillah, which was said uniquely, by falling on their faces. אפיים נפילת, to daven fallen on the face, is considered a very special tefillah and is mesugal for the tefillos to be answered. After we conclude Shemonah Esrei, we ask some requests with Tachanun, and it is a mesugal time and mode of tefillah for the tefillos to be answered.
Rabbeinu b'Chaya (16:22) explains: "Moshe and Aharon prayed fallen on their faces, and this is the source of אפיים נפילת (tachanun) when people fall on their faces to pray."
Rabbeinu b'Chaya says that praying while falling on the face expresses (a) yiras Shamayim, that one is afraid to look at Hashem, Who is before him. (b) It expresses distress and agony because when a person is sad, he often falls on his face. We pray this way to show that we are so upset with our aveiros that we can't even raise our faces. (c) Fallen on the face demonstrates that we depend entirely on Hashem's aid. We can't sit up straight, we can't move, and we can't do anything without Hashem’s help. We are entirely dependent on Him.
"When one falls on his face, closing his eyes and mouth, this demonstrates that he cannot see what can help or harm him, and he doesn’t know how to take care of himself. He is helpless without Hakadosh Baruch Hu's aid. It is like his senses are muted, tied, his eyes and mouth closed, and he can't see or speak unless it is Hashem's will.
"This is also why we pray with our feet together, as though they are tied. This shows that a person can't attain his needs on his own. The gentile nations do something similar; they put their hands together when they pray. They don’t know why they pray that way. But this is the reason. It demonstrates that they don’t have any strength. It is like their hands are tied, and...
