Source of custom of putting on head tefillin before completely winding yad tefillin?

Question

When reading an Egyptian halachic work I noticed that it mentioned two customs for wrapping tefillin. The more general custom was to wrap the tefillin shel yad but leave it incomplete, put on the shel rosh, and then finish tying the shel yad. The other custom was to completely tie the yad before doing the rosh.

Since we are referring to Egypt, it seemed likely that there would be some that would be holding by the Rambam there, so upon looking in the Rambam i found the following:

Halacha 5

When does the above apply? When one puts on only one of them. If, however, one puts them both on, one recites only a single blessing, "...to put on tefillin."

One should tie the tefillin on one's arm, and afterwards put on the head tefillin. When one removes them, the head tefillin should be removed, and then the arm tefillin.

Source: http://www.chabad.org/library/article_cdo/aid/925425/jewish/Tefillin-Mezuzah-and-Sefer-Torah-Chapter-Four.htm/mobile/false

The Rambam seems clear that one puts on the arm [completely] and then puts on the head. So where does the custom of leaving the yad incomplete come from? Is it based on kabbalah?

Answer 1

If I understand what you mean, it's written explicitly in the Shulchan Aruch (OC 25, 11):

אחר שקשר של יד על הזרוע יניח של ראש קודם שיכרוך הרצועה סביב הזרוע

After the tying [of the first main knot] of yad on one's arm, he should put [the tefillin] shel rosh before wrapping [the rest of] the strap around his arm.

It is likely that Rabbi Yosef Caro ruled this way based off of the Tur, in the name of his father, the Rosh (and not the Rambam, who doesn't mention this).

The reason as explained by the Mishnah Berurah (there):

הטעם כיון שברכת להניח חוזרת גם על של ראש כנ"ל בס"ק ל"א וכ"ש להמחבר לעיל בס"ה דס"ל דאינו מברך אלא אחת ע"כ ימעט ההפסק בכל מה שיוכל והכריכה סביב הזרוע אינה מעיקר הדין ע"כ טוב לאחר אותה עד שיניח של ראש...‏

The beracha ("lehaniach tefillin") is for shel rosh as well (even for Ashkenazim who say two berachot), so there shouldn't be an unnecessary pause in between this beracha and putting shel rosh. Therefore, after tying the first knot of yad (which is the main knot), it's time to immediately put shel rosh, and the rest of the yad can be wrapped at the end.

The Mishnah Berurah, Magen Avraham, and more also mention the Ari's custom to wrap the strap seven times on the arm, before putting shel rosh.