Commentaire sur Le Deutéronome 31:10

מקץ שבע שנים AT THE END OF EVERY SEVEN YEARS [AT THE APPOINTED SEASON OF THE SHEMITTAH YEAR] — This means in the first year of the new Shemittah-period — in the eighth year (Sotah 41a). But why does Scripture call it the “Shemittah-year”, which name usually implies the seventh year? Because the law of the “seventh year” still applies to it, viz., in connection with the harvest of the seventh year’s produce that actually goes forth into (takes place in) the year following the seventh year (the eighth year) (Rosh Hashanah 12b; cf. Rashi on Exodus 34:21).

Because there are still seventh-year restrictions which apply, etc. This means if he plants in a prohibited manner during the seventh year, it remains prohibited after the seventh year as it was during the seventh year. Alternatively it refers to produce that grew by itself during the seventh year.