Q. I have seen that when some people stop learning in the beis hamedresh to go temporarily outside, they turn the sefer being learned around, so the upside now faces the top of the table, is that proper?
Others use another closed sefer and place it on top of the exposed written text being learned, in order to cover it, is that correct?
A. Shach and Taz (Y.D. 277: 1) quote Bach and Yerushalmi Megila, that the one who stops learning and goes to another room, leaving an open sefer, will forget his learning. They quote Cabbala sources which teach, that an angel named Shed (the first letters of Shomer Dafim or the one watching over the Torah pages), will become empowered and make the one who so behaves forget the Torah he has learned.
The above is based on Shulchan Aruch (Y.D. 277: 1) which rules that if after writing a page of a Sefer Torah, if the ink is wet, to avoid dust falling on the wet letters while they dry, one should avoid turning around that page around, as this is considered disrespectful and derech bizayon. One should rather leave the wet page up and cover it with a hanging cloth until it dries.
Mishna Berura (154: 31) advises not to use another sefer to cover the first open one.
Horav Shlomo Miller’s Shlit’a opinion is that one leaving temporarily the room should indeed not maintain the sefer being learned open. He should close it (he may leave a page mark to find the place).
If need be, to keep his seat from being taken by others, he may just leave the book cover open.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a