In the Torah, the precept of putting on Tefillin appears on four occasions: twice in the book of Shemot, Exodus, upon leaving Egypt, and twice in the book of Devarim, Deuteronomy, when Moses repeats the Torah to the generation entering the land of Israel (one of them in our Parashah: and you shall bind them as a sign upon your hand and they shall be as frontlets between your eyes)
We notice a clear difference between the way the precept is mentioned on both occasions in the Book of Exodus and the language used by the Torah in the Book of Deuteronomy: In Exodus, the hand tefillin and the head tefillin are cited as part of the same action ("and it shall be for you as a sign upon your hand and as a reminder between your eyes"—"and they shall be as a sign upon your hand and as foreheads between your eyes"), while in the Book of Deuteronomy they are cited as two different entities ("and you shall bind them as a sign upon your arm, and they shall be as foreheads between your eyes"). Why this difference?"
A SPECIAL FORCE
The precept of Tefillin possesses a special power to confront the enemies of Israel. Regarding this characteristic of Tefillin, we find two aspects:
a) Regarding the verse "and all the nations of the world will see that the name of Hashem is proclaimed upon you and they will fear you," our Sages, of blessed memory, say, "this refers to the Tefillin on the head." That is to say, the mere fact that a Jew places Tefillin on his head generates fear and panic among the enemies of Israel.
b) In Halakha, Jewish law, it is written: "For the sake of fulfilling the commandment of Tefillin and making them as prescribed by the Law, it shall be fulfilled in the soldiers of the battle 'and he shall cut off the arm next to the head'." Here the power of Tefillin in the midst of war is expressed, and Tefillin helps to strike the enemy decisively.
TWO ASPECTS
These two aspects are related to the question of whether the hand tefillin and the head tefillin are two separate entities or a single entity:
In the second aspect of the precept of Tefillin, the hand and head tefillin have different functions: The hand tefillin causes the blow to "cut off the arm," while the head tefillin causes the blow to be "also on the head." While in the first effect—to generate fear in the enemy—the action is caused by the head tefillin ("this alludes to the head tefillin"), the hand tefillin only provides additional force to the head tefillin.
WAR THROUGH NATURAL CHANNELS
This explains the difference in how the precept is mentioned in the Book of Exodus compared to the Book of Deuteronomy. At the time of the Exodus from Egypt, when the entry into the Land of Israel was destined to be led by Moses, the conquest of the Land was meant to occur miraculously, supernaturally. In that situation, the Jews would not have needed a war; the head tefillin would have been sufficient to instill fear and dread in Israel's enemies. Therefore, both tefillin, the hand tefillin and the head tefillin, appear as a single entity.
But when Moshe addresses a generation that is ready to enter the Land of Israel knowing that this will be done at the hands of Yehoshua, so the Jews will have to carry out a war by natural means, there he highlights the special ability of the Tefillin of the head, each one with its specific strength in striking the enemy - "and will cut off the arm" and "also the head".
(Likutei Sichot Volume 9, Page 49)