TEFILLIN OF THE HEAD
דברים פרק כח, י וְרָאוּ כָל-עַמֵי הָאָרֶׁץ כִׁי שֵם ה' נִׁקְרָא עָלֶׁיךָ וְיָרְ אוּ מִׁמֶׁךָ:
מסכת מנחות דף לה/ב ר"א הגדול אומר אלו תפילין שבראש
The Tefillin have the majority of G-d’s name, the Shin and the Daled (without the Yud). Tosfos explains that the word לטטפת is a expression indicating power, as if to say that the Tefillin are witness for plus a reproach from HaShem that He is revealed through the Tefillin and people would see the Tefillin and fear G-d. In Brachos, the derasha is made that Tefillin are a source of strength for the Jews. Similarly, the verse in Yeshayahu, G-d swore with His right hand (this is Torah) and with His (other) strong arm (this is Tefillin that is a source of strength to the Jews). In reality, the left hand symbolizes the weak arm. Thus, even though the left hand is not the source of power, yet the left hand is the place of the source of strength (Tefillin which rest on the left arm).
