At the beginning of the Parsha, the Torah seems to go out of its way to inform us that the laws of Shemittah were told on Har Sinai ויקרא כ"ה:א'(). Why is there this emphasis on a point which is so obvious, considering that all of the Mitzvos were given to us on Har Sinai? Many Meforshim answer that it is to inform us that just like we see that even all the intricate details of Shemittah were given at Har Sinai, the same is true of all the other laws given at Har Sinai.
An interesting question arises concerning this interpretation, however, because if even the minute details of all the Torah's laws were indeed given at Har Sinai, exactly where are they written? They are actually not in the Torah itself, because all the laws there are written in general terms, even in relationship to Shemittah; there are in fact many details of Shemittah that are not found clearly in the Torah at all. What do Chazal mean, therefore, when they say that all the details of the laws of Shemittah were given at Har Sinai? The answer is that these laws were conveyed through a different type of Torah, namely, the Torah SheB'al Peh, the Oral Torah, which was handed down by the Sages verbally. All of those laws were given at Har Sinai as well, and the Torah, which specifies this fact with regards to Shemittah, thereby informs us that such is the case with all the Mitzvos of the Torah; both the Written Torah and the Oral Torah were given at Har Sinai.