Covering the Tefillin Shel Yad:
There is no need to be particular to cover the Tefillin Shel Yad [i.e. hand Tefillin], such as with one’s sleeve, and one can choose to wear it either revealed [with his sleeves rolled up on top of the Shel Yad] or concealed [under a sleeve]. [Thus, although often the clothing automatically cover the Shel Yad, there is no need to be particular in this matter. However, some Poskim rule one that must be particular that the Tefillin Shel Yad is always covered, such as under the sleeve. Likewise, according to Chassidus and Kabbalah, the Tefillin is to remain covered throughout the time it is worn. Furthermore, some people, based on Kabbala, are accustomed to cover the bicep using their Tallis [or jacket] upon putting on the Tefillin Shel Yad. The custom of the Rebbe Rashab was to cover the Tefillin Shel Yad with his sleeve, and he would do so even prior to putting on the Shel Rosh. So was also done by other Gedolei Yisrael, to immediately cover the Shel Yad, prior to wearing the Shel Rosh. The Rebbe’s was often witnessed to have his Shel Yad covered with his sleeve, although on occasion, it was seen to be uncovered. Practically, the widespread Chabad custom of previous generations was to be particular to always cover the Shel Yad, although today, for some reason, some Chassidim are particular in this while others are not. It is however a widespread Chabad custom to cover the Shel Yad while saying the blessing, as brought above from the Poskim.]
According to final ruling in Halacha, there is no need to be particular that the Shel Yad remain covered, and one can choose wear it either revealed, with the sleeve rolled up above it, or concealed under his sleeve. However, based on Chassidus and Kabbala, the Shel Yad is to be covered upon saying the blessing, and remain covered throughout the time it is worn. Practically, the widespread Chabad custom of previous times was to cover the Shel Yad with the sleeve, and so was the custom of the Rebbe Rashab, and so is the custom of many Chassidim even today. Others however, are not particular today to cover the Shel Yad with the sleeve, perhaps due to the fact the Shel Yad is in any event covered by the box. Nonetheless, those who do not wear the box over the Shel Yad, are to be particular to cover it with the sleeve, based on the Chassidic custom.
When is one to cover the Shel Yad, before or after putting on the Shel Rosh?
 Admur 27:8 “The obligation to cover the Shel Yad is only in a case when the Shel Yad is being worn over an item which is never usually covered, such as a cloth or bandage, however when it is worn on the actual skin, there is no need to cover it.”; Rama 27:11 “By the Tefillin Shel Yad there is no need to be particular in whether they are revealed or concealed”; Mordechai p. 96; Menachos 37b “Rav Ashi questioned Rav Acha who had his Shel Yad revealed, don’t you hold that the Shel Yad must be a personal sign that is not for others, and he replied it refers to an area which is a personal sign”; M”A 27:21; Machatzis Hashekel ibid; See also Admur 27:23 regarding not covering the head Tefillin and Admur 45:2 regarding entering a cemetery. This law is omitted from Ketzos Hashulchan 8; See Piskeiy Teshuvos 27:23; There is a picture of the Chofetz Chaim printed in which his Shel Yad is not covered. [See Heroes of Modern Jewish thought p. 47]
 The reason: One who is wearing his Tefillin Shel Yad over his clothing due to an injury must beware to cover the Shel Yad so it is not revealed, as the verse states “Vehaya Leos Al Yadeshca” which means to say that the Shel Yad is to be for you a sign, and not a sign for others. However, the obligation to cover the Shel Yad is only in a case when the Shel Yad is being worn over an item which is never usually covered, such as a cloth or bandage, however when it is worn on the actual skin, there is no need to cover it. This applies even if his shirt, and all his clothing, tears in the area opposite the Tefillin Shel Yad. Now, although the Shel Yad can be seen by others, this does not pose any issue, as the Torah was not particular to require the Tefillin Shel Yad to specifically be covered, and rather it may be worn even if revealed, so long as it is placed on an area which is normally covered, which is the bicep. [Admur ibid; See M”A ibid Machatzis Hashekel ibid] This means to say as follows: From the words “Lecha Leos” we learn that the purpose of the Shel Yad is to serve as a sign for oneself and not for others. However, this does not mean to say that the Shel Yad must be covered so others don’t see, but rather that it must be placed on an area that is normally covered, such as the bicep. The fact that it is placed on a normally covered area, suffices to emphasize that it is a personal sign “Lecha Leos”, even if in truth others can see it, due to a torn shirt. In other words, from this Drasha of the Gemara we learn that one may not have a Chatzitza between the arm and the Tefillin. Likewise, when it is placed over a sleeve due to an injury due to lack of choice, it must be covered, as only then does one fulfill the personal sign of “Lecha Leos”, as otherwise, it is placed on an area that is seen by others.
 See Admur ibid “This applies even if his shirt, and all his clothing, tears in the area opposite the Tefillin Shel Yad.” Which implies that usually it is covered by his clothing; See also Admur 45:2 that “It is permitted to enter a cemetery with only the Tefillin Shel Yad, being that it is covered”, thus implying that it is normally covered.
 Orchos Chaim Tefillin 17 and Mordechai Menachos 12 in name of Shimusha Raba, brought in Beis Yosef in end of 27; Levush 27, brought in Birkeiy Yosef 27:5, Shaareiy Teshuvah 27:17, Kaf Hachaim 27:56; See Admur 45:2 that “It is permitted to enter a cemetery with only the Tefillin Shel Yad, being that it is covered”, thus implying that it is to be covered. However, in truth, this is just saying that the Shel Yad is commonly covered by one’s clothing, and not that one must be particular to do so.
 The reason: This is due to the literal understanding of “Lecha Lesos”, that the Tefillin is to be only a personal sign and not a sign for others. [Orchos Chaim Ibid; Shimusha Raba ibid; Beis Yosef ibid; Levush ibid]
 Implication of all sources in next footnote who hold of the reason of “Lecha Los” [see Ishkavta Derebbe p. 36 that they all hold it must be covered even when putting it on and certainly upon wearing it] ; Siddur Im Dach p. 18 [Said by the Alter Rebbe, written by the Mittler Rebbe, and edited by the Tzemach Tzedek] “By the Tefillin Shel Yad it says Lecha Leosand not to others as an Os, from which we learn that the Shel Yad must be covered and not revealed.”
The reason: As the Tefillin Shel Yad represents the investment of the Divine light into the vessels, in which the light is concealed and not revealed. [Siddur Im Dach ibid]
 Birkeiy Yosef 27:4 in name of his grandfather Rav Avraham Azulaiy [the Chesed Leavraham], based on the teachings of the Arizal, brought in Shaareiy Teshuvah ibid; Shaar Hakavanos Derush 5; Peri Eitz Chaim Shaar Hatefillin 10; Olas Tmaid p. 34; Elya Raba 27:5; Shalmei Tzibur p. 39; Zechor Leavraham 1 Os Taf; Beis Oveid Tefillin 28; Kisei Eliyahu 25:2; Chesed Leavraham 25:12; Kaf Hachaim [Falagi] 10:18; Ben Ish Chaiy Vayeira 15; Shulchan Hatahor 27:1; Kaf Hachaim 25:32
The reason: As the Tefillin are to be put on privately. [Poskim ibid] Alternatively, this is due to the Drasha of Lecha Leos. [Maharikash, brought in Ben Ish Chaiy ibid; Kaf Hachaim ibid] Due to the first reason, some of the Poskim ibid write that even the Shel Rosh is to be covered while putting it on. [Gloss in Peri Eitz Chaim ibid; Birkeiy Yosef ibid in name of Rav Avraham Azulaiy; Shalmei Tzibur ibid; Kaf Hachaim ibid]
 Ishkavta Derebbe p. 32; Rabbi Lable Groner related to me that the Rebbe was accustomed to place the plastic box on prior to the SDhel Rosh. See coming footnotes for possible explanation.
The reason: Although it is forbidden to make any interval between the wearing of the Shel Yad and Shel Rosh, nevertheless, since we hold of the Drasha of Lecha Leos Velo Leacheirim Leos, therefore covering the Shel Yad is included as part of the Mitzvah of wearing it, and is hence not considered an interval. [Ishkavta Derebbe ibid]
 See Orchos Rabbeinu; Kovetz Shefa Kodesh ; Zechor Leavraham; Piskeiy Teshuvos ibid
 From a look in videos and pictures, it appears covered by the sleeve in many of the shots. Rabbi Lable Groner however stated to me that while this was case in many instances, not always did the Rebbe’s sleeve cover his Shel Yad. It is strange that there are those who say that the Rebbe never covered his Shel Yad. In any event, it is unclear if in general the Rebbe was particular to cover the Shel Yad and only on occasion did it slip out, or if the Rebbe was not particular in this matter at all, and let the sleeve make the decision. [When wearing short sleeves, it often on its own can cover the Shel Yad.] Vetzaruch Iyun; However, see next footnote!
 Ishkavata Derebbe ibid; See also Toldos Levi Yitzchak 1 p. 138
 Perhaps one can suggest that the widespread custom of today to place a box on top of the Shel Yad [which was not followed in previous times, and was not done by the Rebbe Rashab-see Ishkavta Derebbe footnote 23] itself suffices in place of covering the Shel Yad, and hence we no longer need to be particular to also cover it with the sleeve, as did the Rebbe Rashab. This can also explain why the Rebbe was accustomed to cover his Shel Yad with the box prior to putting on the Shel Rosh [heard from Rav Lable Groner], as the box fulfilled this necessity, and thus does not serve as an interval between the Shel Yad and Shel Rosh. [unlike the ruling in Piskeiy Teshuvos 27:23] Likewise, the Rebbe was not particular to always cover the Shel Yad also with a sleeve, being it was already fulfilled with the box.
 Ishkavata Derebbe ibid
The reason: See previous footnotes!
 Heard from Rav Lable Groner; unlike Piskeiy Teshuvos 27:23
The reason: See previous footnotes!