Or HaChaim on Exodus 13:16/אור החיים על שמות י״ג:ט״ז

1‎[This is a comment to the Torah’s‎‏ ‏statement, “It shall be a sign upon your Handידכה ‏‎.”] Our Rabbis ‎of blessed memory said (Menachot 37a), “’Yadchah (spelled with an extra hey implies the weaker ‎hand (yad kehah), which is the left hand. We need to understand why Hashem did not choose the ‎right hand to perform this mitzvah, since it is greater. Our Rabbis say that this is in order that (the ‎tefillin) be aligned next to the heart, which is towards the left side (of the body), and their words ‎are true. However it appears to me that there is an additional reason, for the Torah itself gives a ‎reason for the matter in saying, “Because with a strong hand.” You need to know that HaShem has ‎‎2 aspects within His attributes. The first is called “the great hand, yad hagedolah”, and the other is ‎called “the strong hand, yad hachazakah”. “The great hand” represents (G-d’s) aspect of kindness ‎and goodness; “the strong hand” represents His strength, which He repays the wicked according to ‎their wickedness. In removing Bnei Yisrael from Egypt, Hashem stretched out “the strong hand” ‎and struck His enemies with ten plagues. For this reason G-d said that tefillin, which contain the ‎written remembrance (of these events), should be placed on our weak hand (our left hand), which‏ ‏represents “the strong hand” that‏ ‏removed us from Egypt. This is why the‏ ‏pasuk here says, “for ‎with the strength of a hand.” This is correct. Granted that it says in many places (regarding ‎punishment), “the great‏ ‏hand,” in those instances G-d’s attribute‏ ‏of mercy concurred with the ‎attribute of justice, but the essence of justice is still‏ ‏performed through the “strong hand.”‎‏ ‏‎{Ed. ‎Note - Thus, despite the term‏ ‏‎“strong,” the “strong hand” which‎‏ ‏represents justice is, in fact, ‎weaker‏ ‏than the “great hand” which‏ ‏represents mercy. To symbolize the‏ ‏vengeance that G-d ‎exacted from‏ ‏Egypt through His “weaker hand,” we‏ ‏don our tefillin, which memorialize‏ ‏these ‎events, on our weaker hand.}‎