Rabbi Yoseph Qafahh wrote in his Commentary
on Maimonides' "Mishne Torah" (Hilkoth Tefillah 5:14), which I will
presently quote for you:
"ממה שסתם רבנו כאן השתחויה כיצד וכו', ולקמן באר מה היא השתחויה, משמע דמסתמא
היא בפשוט ידים ורגלים, אע"ג דלקמן כתב יש מי שעושה קידה, ומ"מ לא באר היאך היא
הקידה, ובודאי לא מדובר בקידה דרשב"ג ודלוי בסוכה דף נג והכא. וכך תאר רס"ג (רבינו
סעדיה גאון) קידה זו וז"ל (וזה לשונו) בסדורו עמ' כד: ותאור כריעתו,
שיניח ברכו השמאלית על הארץ כאשר הוא יושב
ומקפל ברכו הימנית עליה כדרכו כשהוא רובץ ויהיה כאלו חציו רובץ וחציו יושב, ע"כ.
ולשון זה הועתק בכל סדורי תימן העתיקים וכך היינו נוהגים לעשות בנפילת אפים
כמסורת הגאונים."
So, here you have it, in the language of Rabbi Yoseph Qafahh and RSG, that
when they made the "Nefillath Apayim," it was NOT with both knees on the
ground, and bending forward with one's face on the ground, as the Arabs will
do in their prayers. Rather, the Yemenite Jews went down into
a sitting posture after standing in prayer,
with one's weight being put on his left knee, while his right knee was bent
over his left, and he tilted towards his left, while burying his head within
the bend of his left arm, in order to make the supplications known as
"Tahanunim."
Mahari"s on nefilath apayim:
The practice of falling upon one's face (Nefillath Apayim) directly after standing in prayer in the morning and in the afternoon was always done in Yemen after the manner described in the Siddur (Prayer Book) of Rabbeinu Sa'adia Gaon. So writes Rabbi Yoseph Qafih in his Commentary on Maimonides' Mishne Torah (Hilkoth Tefillah 5:14), as also Rabbi Yoseph Subeiri in his book, "We-yitzbor Yoseph Bar," volume 1. Since synagogues in Yemen were bare of chairs and furniture, the congregation sat upon cushions strewn across the floor against the wall, and when it came to making "Nefillath Apayim," they simply went down into a sitting posture, and then inclined their bodies to their left side, with one's full body weight and pressure being brought to bear on his left knee and thigh, while his right knee was bent over his left. This, as Maharitz describes in the following excerpt, was done while clasping one's left arm with his right. A man's face and head were entirely covered beneath his shawl (talith) while doing this, and held in the bend of his left arm. "…And when he falls upon his left [side], left him not fall down upon his hands themselves, but rather opposite his arm. That is, let him clasp [his] left arm with his right, and fall upon his left [side]. (Cf. book "Or Hayashar" ) And know assuredly that there is no difference between Nefillath Apayim on one's left side in the morning [prayer] and in the afternoon [prayer]. A man ought always to do so, unlike Rabbi Moshe Iserlische, and this is our custom, as well as what appears to be the opinion of our teacher, [even] the Rabbi, Yoseph Karo, and the expositors of our laws."