Why does the Torah see fit to state "Sheish Shanim Tizra Sadecha ... "?
Ramban #1: It is the way the Torah normally speaks. 1
Ramban #2 (citing Yerushalmi in Kil'ayim): It comes to teach us that someone who works his land in the Shemitah transgresses an Asei 2 (as well as the La'av in the following Pasuk.
Ramban #3 (citing the Mechilta in Mishpatim) and Seforno: When Yisrael do the will of Hashem they experience one Shemitah (per seven-year years), as the Torah writes "Sheish Shanim Tizra es Artzecha ... "; but when they don't, they have to keep four Shemitos - by leaving the land fallow in the first, third and fifth years, 3 followed by the Shemitah in the seventh.
What are the implications of "Ve'asafta es Tevu'asah"?