http://revach.net/article.php?id=3741

Section: Parshas HaShavua   Category: Quick Vort
Parshas Behar: Ben Ish Chai - What Do Har Sinai And Shmita Have In Common?
"Vayidaber Hashem El Moshe B'Har Sinai" (Behar 35:1).  Mah Inyan Shmita Eitzel Har Sinai?  Rashi famously asks why does the Torah say that Shmitta was given on Har Sinai.  Rashi answers that this teaches us, that just like Shmita, which was not mentioned in the Aseres HaDibros, was given on Sinai, so too were all the other mitzvos.  But why is this lesson taught by Shmita as opposed to any other mitzva?

The Ben Ish Chai answers that Sinai and Shmita had a fundamental attribute in common.  They both symbolize that "less is more."  Har Sinai was chosen to host Matan Torah because it was the lowest and humblest of mountains.  Its lack of greatness enabled it to be the site of the greatest event in history.  Similarly, Shmita is a year where we allow the field to remain barren.  We do not help it produce any fruit.  Yet, Hashem promises that this will spark an abundant crop like no other, leading to yields of two to three years, from a single bumper crop in the sixth year.

For this reason, says the Ben ish Chai, Shmita was the one mitzva that Hashem used to illustrate that the entire Torah was given at Har Sinai.