Lubavitchers generally do not wear the tallit katan over the shirt like other Chasidim. Is this because there is some halacha about it, or kabbalistic preference, or is it simply because the Rebbe did so like other if not most Lubavitch levush minhagim.
The Shulchan Aruch (Orach Chaim 8:11) writes:
עיקר מצוַת טלית קטן ללובשו על בגדיו, כדי שתמיד יראהו ויזכור המצוות
The main Mitzvah of Tzitzis is to wear it on one's garments, so one would see them and remember the commandments.
But the Magen Avraham writes in s"k 11:
ובכתבים, דטלית קטן דווקא תחת בגדיו, עיין שם In the writings (the Pri Eitz Chaim) it says that the garment should be specifically under the garment, see there for the reason.
The Lubavitcher Rebbe (in Likkutei Sichos v. 33 parshas Shlach) wrote that according to Kabbalah, both the garment and the strings should be concealed.
The Alter Rebbe wrote in the Shulchan Aruch HaRav 8:18-8:19
יח וכיון שטעם לבישת הציצית הוא כדי לזכור המצות לפיכך הלובש טלית קטן יזהר ללובשו על בגדיו כדי שתמיד יראהו ויזכור המצות ואותן הלובשים אותו תחת הבגדים יזהרו שילבשו אותו באופן שיראו הציצית תמיד שע"י ראייתן יזכרו המצות תמיד:
יט צריך שיהיו הציצית מבחוץ שנאמר וראיתם אותו ודלא כאותן שתוחבין אותן בהכנפות ובשעת הדחק כגון שהולך בין העכו"ם וחושש שלא ילעיגו עליו יכול לתחבם בהכנפות. אך בשעת הברכה יהיו מגולין לפחות כדי הילוך ד' אמות:
Since the reason for wearing Tzitzis is to remember the commandments, therefore one who wears a Tzitzis should be careful to wear it over his garments, so that one will always see them and remember the commandments. And those that wear them under their garments should wear them in such a way that he'll see his Tzitzis constantly and he'll constantly remember the commandments.
One has to be careful to wear his Tzitzis outside, as it says ""And you will see them", and not like those who tuck them in. And if one has no choice, such as if one walks among the non-Jews who he suspects will make fun of him, he can tuck them in.
But since the Shaar Hakollel writes in the name of Reb Hillel of Paritch that the Rabbi Shneur Zalman of Liadi said that whenever there's an argument between the Mekubalim and the Poskim, one should follow the Mekubalim. So, in contrast to his ruling in his Shulchan Aruch, he would agree that one should wear theme under his garments.
So why not wear the strings under the garments (and go fully Kabbalistic) ?
The Lubavitcher Rebbe answered that it could be that there are two ways to look at the commandment to wear Tzitzis.
- One has to wear strings on his four-cornered garment. And it helps one remember the commandments.
- One has to wear strings on his four-cornered garment in order to help one remember the commandments.
The Shulchan Aruch (both the original and the Shulchan Aruch HaRav) follows the second way, so if one wears Tzitzis in a hidden way, he doesn't (or is close to not) fulfilling the Miztvah of Tzitzis, while the Arizal follows the first approach.
So again, why not wear them under the garment?
Because, as the Lubavitcher Rebbe answered, the Arizal never said that people shouldn't wear Tzitzis under the garment. It's just that he wore Tzitzis under his garment.
The reason is that the Arizal didn't need the simple reminder of Tzitzis (he wouldn't violate the Mitzvos anyways), so he was able to wear it under his garment (because he needed Tzitzis to remind him of the inner dimension of Mitzvos), while the rest of us, who are liable to outright sin, have to wear it above their garments.
So why don't we follow the Shulchan Aruch and wear it above our garment?
As a reminder to us, about the lofty service of the Arizal and the likes.