Why do I hide my face in my arm? Did people really lie on the floor?
Yes, and Tanach (the Hebrew Bible) describes it. After the golden calf, Moshe says he threw himself down before God for forty days and forty nights (Devarim 9:25). After an army defeat, Yehoshua (Moshe's student and successor) lay on his face before the Ark until evening (Yehoshua 7:6). The Rambam (Maimonides, the great medieval law-writer, died 1204) describes the same thing after the Amidah: you sit, then fall on your face to the ground and say whatever pleas you want.
But the Gemara already had a worry. Rabbi Elazar taught that an important person may not fall on his face unless he is sure to be answered, like Yehoshua (Taanit 14b). Notice how God's answer to Yehoshua began: "Get up. Why are you lying on your face?" (Yehoshua 7:10).
The Tur (a law code from the 1300s) adds a limit from the Yerushalmi (the Gemara of the Land of Israel). The worry applies to someone praying for the community, in public. If he is not answered, people start to think he must not be worthy. Alone, it is fine.
Rashi (the main medieval commentator on the Gemara, around 1100) uses this rule to explain a line about two famous teachers, Abaye and Rava. They prayed "leaning," on their sides, and did not really fall on their faces, because an important person may not.
The Tur also brings Abaye and Rava as the model for leaning to one side, though his reason is different: the Sages forbade lying face down on a stone floor. Either way, leaning is a smaller version of falling. As for what the gesture is for, Peninei Halakhah (a modern guide to Jewish law) says it shows a broken heart, with the body made small and the self handed over.
Devarim 9:25,Yehoshua 7:6 and 7:10,Mishneh Torah, Laws of Prayer 5:13,Taanit 14b,Tur, Orach Chaim 131,Rashi on Megillah 23a,Peninei Halakhah, Prayer 21:2Do I have to say this at all, or is it extra?
Originally, it was extra. Peninei Halakhah says the Sages never set up falling on the face as a required prayer, and never gave it a fixed text. Anyone who wanted to could add their own pleas after the Amidah. The book guesses that the freedom is part of the point: a prayer of total surrender should come from the heart, not from a script.
The Tur reports that Rav Natronai Gaon (a head of the Babylonian Jewish academies, over a thousand years ago) called falling on the face after the Amidah optional. Peninei Halakhah adds that this is why people skip it in the house of a groom.
A second proof comes from Ima Shalom. Rabbi Yitzchak Ibn Giat (a Spanish Torah scholar of the Middle Ages) argued that if it were required, she could not have stopped her husband from saying it. The story is on the first story card.
Over time, the text did become fixed. Peninei Halakhah says that in the time of the Geonim (the heads of the Babylonian academies) a set wording began to form. In the centuries after, it settled, until Jews took on a set text of supplications as an obligation. The book's guess about why: the hardships of exile dulled hearts, so a fixed text was needed.
So the custom today is strong, but the law keeps some give. Peninei Halakhah quotes later authorities who say that when there is real doubt whether to say it, it is better not to, since it is voluntary.
Peninei Halakhah, Prayer 21:2,Tur, Orach Chaim 131,Bava Metzia 59bWhat if I say this and do not mean it? Is that dangerous?
A passage in the Zohar (the central book of Kabbalah) makes this a real worry. It describes this moment as giving your soul to God. At night, it says, you leave your soul with God for safekeeping. Here it is not a deposit. You hand it over, like someone who has really left the world. (The Sparks for Adon Olam explain the nightly deposit.)
Then the Zohar warns. Someone who does this with a heart far from God is not accepted. A voice announces that he "will not be established before My eyes" (Tehillim 101:7). It adds a harsh line: such a person causes himself to be taken from the world before his time.
That frightened many readers, and the Kaf HaChaim (a large book of Sefardi law, printed in Jerusalem in the early 1900s) answers it directly. Reading the Zohar together with the writings of the Arizal (Rabbi Yitzchak Luria, the great Kabbalist of Tzfat), he says the danger applies only to a person who aims at a special mystical goal in this moment. If you say it as a plain request for mercy, he writes, there is nothing to be afraid of.
He still asks you to try. Pay attention to the meaning of the words. And he says every person should say this psalm every day, as the Zohar and the Arizal teach. Like the rest of prayer, it has deep layers most of us do not reach, and we say it anyway, trusting God's mercy.
One detail: the psalm the Zohar and the Kaf HaChaim mean is Tehillim 25, not the Tehillim 6 in your siddur. Both are said at the same moment, when you fall after the Amidah.
Zohar, Bamidbar 8:73-76,Kaf HaChaim, Orach Chaim 131:31