If the Yud shel kesher is not touching the bayit shel yad in tefillin, can someone use a rubberband or a twisty to keep in place? What is the source for this halacha? Is it kabblah? How important is it? Is one yotzei?
The source for this halacha is the Zohar (Pinchos 232b), and is brought in O:CH 27-2, and M:B 27-10. There are even those who say that according to the Zohar, the yud of the shel yad should not move at all from touching the shel yad even when the tefillin are not being worn. According to the Zohar it is very important and this is what should be done. In retrospect if it wasn’t done, according to Piskei Teshuvos (27-6) if one realized after realized after removing his tefillin that the yud wasn’t touching the bayis, he doesn’t have to put them on again, since there are opinions that if the yad is touching the titura (bottom part of the tefillin) that is also sufficient. Other than that, one should be careful to make sure that it does indeed touch.
It is permitted to use a rubber band to keep the yud attached, however R’ Eluyasiv held that it is better to specifically use a “gid”.