The experience of mattan Torah was one of synethesia, "ro'im es hakolos," Bnei Yisrael saw the voice of Hashem. Miracles have a purpose -- there must have been some reason for Hashem's voice to be not only heard, but also seen.
There is a famous story in which R' Chaim Volozhiner was recounting the greatness of his brother, R' Itzele, and remarked that he would be considered a talmid chacham even had he lived in the days of Amoraim and even Tanaim. The listeners were fabergasted, as surely R' Chaim Volozhiner was not given to saying empty praises. If this is how R' Chaim described his brother, what could he possibly say about his rebbe, the GR"A? R' Chaim Volozhiner explained the difference between the two as follows: if you ask a Jew to tell you the previous word in any pasuk in Ashrei, or the previous word in a sentence of Aleinu, he will know the answer in a second, but to get that answer he will have to say that pasuk or sentence in his mind from the beginning. R' Itzele knew kol haTorah kula by heart like Ashrei or Aleinu. The GR"A was different -- he knew the previous word without having to think of the pasuk from the beginning; he knew kol haTorah kula backwards as well as forwards.
What's the difference between these two types of knowledge? When asked for the previous word of a pasuk in Ashrei by heart, we rely on our auditory memory, and therefore have to recite the pasuk from the beginning. If we have a sidur open in front of us, we just look at the previous word and can answer without hesitation. The former is shemiya, the latter is re'iya. Kabbalas haTorah was an experience during which Torah was absorbed and accepted to the greatest degree humanly possible. That type of knowledge comes only through re'iya, seeing, not just shemiya, hearing.
Our parsha opens with the laws of the even ivri, the Jewish slave. If the slave chooses to remain indentured beyond six years, his ear is punctured. Why? Rashi cites from Chazal: "The ear which heard on Sinai that Bnei Yisrael are my [Hashem's] servants and not servants to servants," deserves to be punished. "Ain lecha ben chorin elah ha'osek baTorah" -- true freedom is through kabbalas haTorah, through learning Torah. A person who wishes to remain enslaved demonstrates that his kabbalas haTorah was done only with his ears, by hearing, but not by seeing -- his experience of kabbalas haTorah was lacking in some crucial element.