Why does the Torah insert this (seemingly superfluous) Pasuk?
Ramban #1 and Moshav Zekenim #1: It is the way the Torah normally speaks. 1
Ramban #2 and Moshav Zekenim #2 (citing the Yerushalmi in Kil'ayim): To teach us that someone who works his land in the Sh'mitah transgresses an Asei 2 (as well as the La'av in the following Pasuk.
Ramban #3 (citing the Mechilta in Mishpatim) and Seforno: To teach us that, when Yisrael do the will of Hashem they will experience one Sh'mitah (per seven years), as the Torah writes "Sheish Shanim Tizra es Artzecha ... "; but if they don't, they will have to keep four Sh'mitos - by leaving the land fallow in the first, third and fifth years, 3 followed by the Sh'mitah in the seventh.
Moshav Zekenim: It is written after "Shabbos la'Hashem" to teach us that if we observe Sh'mitah properly, we will sow our fields; if not, we will have to sell them (and subsequently, also our houses and our daughters - Kidushin 20a).
Oznayim la'Torah: To highlight the miracle that, when one keeps the Sh'mitah, one will be able to to till the land for six consecutive years - as opposed to the normal system, where the land is left fallow every second year. 4
Yerushalmi Kil'ayim, 5:1: Bearing in mind the "Veshavsah ha'Aretz Shabbos la'Hashem" - in Pasuk #2, it teaches us ('Im Eino Inyan') that all other Melachos - besides planting and pruning, are only subject to an Asei and are not therefore sibject to Malkos. 5
Why does the Torah repeat the words "Sheish Shanim"?
Oznayim la'Torah: To teach us that the time that one is prohibited from plowing a cornfield (until Pesach) differs from the time that one may not plow a foeld of trees (until Shavu'os). 1
See Oznayim la'Torah, DH 'Sheish Shanim' #1.
What are the connotations of "ve'Asafta es Tevu'asah"?
Seforno: It teaches us that, if we observee the Sh'mitah, the land will yield its crops for si consecutive years, unlike the norm, where, if one plants an area of land consecutively, it loses its strength and stops producing.
Rashbam: It means that, during the first si years of the Sh'mitah cycle, one is permitted to gather the produce of one's own field and not declare it Hefker. 1
Yerushalmi Ma'asros, 5:2: Based on its jutaposition to "u'va'Shanah ha'Shevi'is", it permits harvesting produce in the Sh'mitah that attained one third of its growth in the sith year. 2
