|Art: Alex Levin|
"And all the nations of the earth will see that Hashem's Name is displayed upon you, and they will revere you" [Ki Tavo 28:10]
In Maseches Berachos [6a], Chazal expound on the above verse: "From where do we know that tefillin are a source of might for Israel? - From the verse "And all the nations of the earth will see that Hashem's Name is displayed upon you, and they will revere you". And it was taught: R' Eliezer HaGadol said "these are the tefillin of the head [sheba' rosh]."
The Sha'agas Aryeh [R' Aryeh Leib from Metz] was once travelling. Throughout the trip he wore his tallis and tefillin and engaged in Torah study.
The wagon driver was also wearing his tallis and tefillin - he was praying while steering the horses.
Suddenly, a band of armed robbers jumped out from the forest and attacked the wagon; they demanded that the driver stop the wagon and hand over all of his money.
The driver was terribly frightened. "Rebbe" he screamed, "we're in danger!"
The Sha'agas Aryeh heard the screaming coming from up front, so he stuck his head out the window to see what was happening. But when the ordinarily bold thieves took one look at the Sha'agas Aryeh, they were overcome with fear and immediately fled.
"Rebbe" asked the driver, "I am both younger and stronger than you. Yet it was you whom the thieves were terrified of. Why did the robbers run away when they saw you?"
"The robbers did not run because of my strength" answered the Sha'agas Aryeh, "but because of the tefillin on my head!"
"But I am also wearing tefillin" responded the driver. "Why did the thieves not run away from me?"
The Sha'agas Aryeh explained: "The Torah says "and all the nations of the earth will see that Hashem's Name is displayed upon you, and they will revere you". The Gemara quotes R' Eliezer HaGadol who teaches that this verse is referring to the tefillin of the head [sheba'rosh]. If you read the words of Chazal carefully, you will notice that Chazal did not say "tefillin she'al ha'rosh" [tefillin that rest on one's head]; rather "tefillin sheba'rosh" [tefillin that are inside one's head]. The holiness of the tefillin must be absorbed into one's head, and then it instills fear into the nations of the world. But if the tefillin are simply lying on top of one's head, then the nations of the world do not fear us at all.
Source: Rabbi Yisrael Bronstein