When Moshe left Har Sinai, it says Vayifen, he turned and went down.
The halacha is that when you leave a holy thing or person you should not turn your back on it, but instead should walk out backwards.
Applications of this halacha.
Leaving a Shul, walking down from the Aron Kodesh, leaving a rebbi, what Reb Meir Simcha says about duchenning, and Rav Hutner's subtle hint.
If so, why did Moshe turn his back?
An alternative interpretation from Rabbeinu Chananel and Ibn Shuib and the Alshich.
The Mishna in Yoma says that the Kohen Gadol would walk backwards out of the Kodesh Kadoshim, facing inwards towards the Aron as when he walked in. The Gemara (53a) goes like this:
וכן כהנים בעבודתן ולוים בדוכנן וישראל במעמדן: כשהן נפטרין לא היו מחזירין פניהן והולכין, אלא מצדדין פניהן והולכין
וכן תלמיד הנפטר מרבו לא יחזיר פניו וילך, אלא מצדד פניו והולך
כי הא דר' אלעזר כד הוה מיפטר מיניה דר' יוחנן, כד הוה בעי ר' יוחנן לסגויי הוה גחין. קאי ר' אלעזר אדוכתיה עד דהוה מיכסי ר' יוחנן מיניה. וכד הוה בעי ר' אלעזר לסגויי, הוה קא אזיל לאחוריה עד דמכסי מיניה דרבי יוחנן.
רבא כד הוה מיפטר מיניה דרב יוסף הוה אזיל לאחוריה עד דמנגפן כרעיה ומתווסן אסקופתא דבי רב יוסף דמא. אמרו ליה לרב יוסף הכי עביד רבא! אמר ליה יהא רעוא דתרום רישך אכולה כרכא
This is brought in the Rambam in 7 Beis Habechira 4 regarding the Beis Hamikdash and 5 Talmud Torah 6 regarding one's Rebbi:
וכשיפטר מרבו לא יחזור לו לאחוריו אלא נרתע לאחוריו ופניו כנגד פניו
and in the Aruch HaShulchan YD 242:43, where it says
As for walking out of shul, the Mechaber does not say that this is necessary, but the Magen Avraham (end of 132, alluded to but not brought explicitly by the Mishna Berura SK 18) says that the same does apply to one who is walking out of a shul or walking away from the Aron Kodesh platform. כשיצא מבית הכנסת לא יצא ואחוריו להיכל אלא יצדד וכן בירידתו מהתיבה. (Walking backward out of shul is not common in the Litvishe community and I did not see this by my Roshei Yeshiva. I have seen it mostly among Sefaradim. Chasidim also do this when leaving a Rebbe, and theoretically it applies to Litvaks too. Rav Hutner, I'm told, had a buzzer on his desk that unlocked the door, and when someone left him, he wouldn't unlock the door until the person realized that he was supposed to walk out backwards and turned to face Rav Hutner.)
Getting back to our parsha:
If it is disrespectful to turn one's back when leaving a holy place or person, and it is a Torah value to avoid doing so, asks Rabbeinu Bachay, why does it say that Moshe turned away and walked down from Har Sinai? Rabbeinu Bachaya says that he turned away because he had to run down to see what was going on with the Eigel, and at times like that, every moment counts, and one should not spend the time on protocol.
A possible problem with Rabbeinu Bachay's answer is that the Torah uses the same expression regarding the second luchos in this parshah as well, in 10:5, and there was no eigel the second time.
The Yalkut Shimoni brings a Medrash that Vayifen means he turned to the side, not that he turned away. Dr. MZ pointed out that we find vayifen used that way by Moshe and the Mitzri. Of course you might argue, as did LW, esq., that the fact that the Torah had to modify it by the Mitzri implies that an unmodified vayifen means to turn away, not turn to the side. I think that looking for linguistic proofs is futile, because it can say וַיִּפֶן בִּנְיָמִן אַחֲרָיו, (Shoftim 20:40), and it can say וַיִּפֶן כֹּה וָכֹה (Shemos 2:12.) Also, why did the Torah have to mention his turning at all? Just say that he left and went down to the people without saying vayifen. There's got to be some chiddush or some mussar haskeil in Vayifen.
The Netziv here and in Ki Sisa, Shimos 32:15, says that of course Moshe didn’t turn his back. But since he was carrying the luchos and walking down a mountainside, he was afraid he would fall down if he walked backwards, as happened to Rava in the last part of the Gemora in Yoma I quoted above, to the effect that he was full of bruises. So the Netziv learns that although Moshe did walk down backwards, the word Vayifen here means that he turned to the side to make sure he wouldn't fall over something behind him.
I mentioned to my shiur that the Rogotchover asks why in 9:15 it says that Moshe was carrying the Luchos in his hands (וָאֵפֶן וָאֵרֵד מִן הָהָר וְהָהָר בֹּעֵר בָּאֵשׁ וּשְׁנֵי לוּחֹת הַבְּרִית עַל שְׁתֵּי יָדָי), while in 10:5 it does not say that Moshe went down with the luchos in his hands (וָאֵפֶן וָאֵרֵד מִן הָהָר וָאָשִׂם אֶת הַלֻּחֹת בָּאָרוֹן אֲשֶׁר עָשִׂיתִי וַיִּהְיוּ שָׁם כַּאֲשֶׁר צִוַּנִי), and he answers that in 10:5 it's talking about the second Luchos, and the second Luchos were given to Moshe Rabbeinu on Yom Kippur, and Moshe couldn’t carry the luchos down, since Har Sinai, a teil hamislakeit, was a reshus hayachid, and the machaneh was a reshus harabbim. (Yes, there is a good question about chai nosei es atzmo, but no better a question than Masa Bnei Kehas). If so, there would be no reason for the Netziv's “vayifen” in 10:5. The kashe was just for fun, because the Rogotchover and the Netziv are learning completely different pshatim.
This just in:
First let me tell you the back story. When I discussed this with my son in law, he said that it's possible that ואפן means that he remained facing the Ribono shel Olam. I firmly disagreed, and said that if that's true, then Klal Yisrael must have walked backwards in the Midbar, because it says פְּנוּ וּסְעוּ לָכֶם. Furthermore, I said, if that's what it means here, why does the Gemara in Yoma 53 have to derive a convoluted proof for walking backwards from the passuk by Shlomo Hamelech, when it is befeirush by Mattan Torah. The best thing I said was that such a pshat is מגלה תורה בפנים שלא כהלכה. (That line doesn't say what you think it says. You have to say it out loud in order to appreciate it.)
And then, Eli sent me a mareh makom to Rabbeinu Chananel and Ibn Shu'ib (last paragraph on the first page and the following paragraph on the next page), who say exactly what my son in law suggested. The Ibn Shuib is here on page 20 in the right column, or here. And worse yet, they say that it is a drasha- not just that this is pshat, but מלמד that he walked backwards. Bishlema if they just said that's pshat, I would say that only after we learn the concept of kavod from the passuk by Shlomo Hamelech do we understand that when it says Vayifen by Moshe, it must mean that he faced Hashem while leaving. But they say מלמד, that you learn from here, that it's a drasha. This is incompatible with our Gemara in Yoma. If this is a drasha, it's a better drasha than the passuk by Shlomo Hamelech and the Gemara should have brought this passuk. I'm not asking this as a kashe. It's not the first time one of the mefarshei hatorah says different than a Gemara. I'm just pointing out that this pshat is definitely not how our Gemara learned, so it does not answer our basic question.
(Rabbeinu Yehoshua Ibn Shuib is a talmid of the Rashba and the rebbi of the Tzeida Laderech and one of the early mekubalim. Drashos al Hatorah is the sefer he is best known for.)
Having seen these mefarshim, I followed up on my son in law's claim that the Targum Yonasan says this, too. I looked at Shemos 32:25, and the Targum Yonasan says ואתפני ונחת משה מן טוורא, which is just another way of saying pana, which proves nothing. But here, the Targum Yonasan says וכוונית ונחתית מן טוורא. I don't know what he changed pshat in pana from Shemos to Devarim, and I don't know if he means to say anything special with וכוונית.
Then he mentioned that he believed that the Alshich says the same thing, and he's 100% right. The Alshich is in Ki Sisa, here. He says that the Medrash Rabba in Shemos perek 12 says this is the pshat in the Vayifen from Pharaoh in Parshas Bo, but it is not in our Medrashim. Eli points out that the Alshich was preceeded by the Ibn Ezra who brings this from a R' Yeshua in Shemos 10:6. Eli says that the word on the street is that this R' Yeshua was a Karaite. Nevertheless, the Ibn Ezra does quote him.
In any case, as I said, these pirushim are contrary to Shas Bavli, and so have no effect on the discussion above. שבעים תורות לפנים.