When must one fast when the Retzuah of his Tefillin flips over?
What is the meaning of the Middas Chassidus (שו”ע אדה”ז סי’ כז ס”ק י”ט) to fast if the retzuos flip over? Does it mean even if it was just flipped over momentarily (like right when it was put on) and he right away corrected it or is it only if he only realized after he took them off and he wasn’t able to put them back on properly, meaning that on that day he only wore tefillin with overturned retzuos? (If it means even if it only flipped momentarily, then why is it such a big deal, any different than the tefillin falling out of place for a second?)
Either when putting it on and it was not turned over properly before one spoke or made a hefsek, or it got turned over at a later point even momentarily.
This only applies to the part of the strap which surrounds the head, and the hand strap only to the first circuit surrounding the arm.
Some say it is only an issue if the strap was lying that way all day long or at least it was this way until after the time of reciting the Shema.
As for the reasons why specifically in the case that the retzuos flipped over there is a midas chassidus to fast (or redeem the fast with tzedakah) please see the 2nd line of the sources.
ראה שו״ע או״ח כז, יא. אדה״ז שם יט. ובשאר נו״כ השו״ע. וראה א״א מבוטשאטש שם.
כף החיים סי’ כז ס”ק מח, אלי’ רבה שם ס”ק נג.