The Tefillin of the head has the letter Shin ש on right and left sides. On the right we have the Shin with three ‘heads’ to represent the patriarchs: Chessed, Gevurah, and Tiferet. The left side has the Shin with four heads to complete the seven Sefirot, connecting the patriarchs to Malchut.
The straps of the Tefillin form the letter Dalet ד on the head and the letter Y י on the hand. Together we get the name שדי, a name that we find on the Mezuzah for our protection. שדי is the name of God for Yessod that funnels the Light to the world.
The Tefillin of the left arm are the aspect of Malchut and the lower H ה of the name.
“נִשְׁבַּע יְהוָה בִּימִינוֹ, וּבִזְרוֹעַ עֻזּוֹ: אִם-אֶתֵּן אֶת-דְּגָנֵךְ עוֹד מַאֲכָל, לְאֹיְבַיִךְ, וְאִם-יִשְׁתּוּ בְנֵי-נֵכָר תִּירוֹשֵׁךְ, אֲשֶׁר יָגַעַתְּ בּוֹ”
“YHVH has sworn by His right hand and by His strong arm, “I will never again give your grain as food for your enemies; Nor will foreigners drink your new wine for which you have labored.”
The Zohar explains that the ‘Right hand’ of God is the Torah and the ‘strong arm’ is the Tefillin.
“וְהָיָה לְאוֹת עַל יָדְכָה וּלְטוֹטָפֹת בֵּין עֵינֶיךָ כִּי בְּחֹזֶק יָד הוֹצִיאָנוּ יְהוָה מִמִּצְרָיִם”
“It shall be as a mark (also sign, letter) on your hand or frontlets between your eyes, for by a strong hand YHVH brought us out of Egypt.”
The word for ‘your hand’ is normally spelled ‘ידך’ and in this verse related to the precept of wearing Tefillin it’s spelled with extra H ה , ‘יָדְכָה’. It’s the Hה of Malchut that we connect to when wearing the Tefillin.