The Torah says (Behar 25:20-21), "If you will ask what shall we eat in the seventh year if we have not planted or gathered our produce, I will direct my blessing to you (i.e. to your land) in the sixth year, and it will provide for the three-year period." The Torah promises those who observe the laws of shemitah that the land will produce double or triple its normal yield in the sixth year, and that will last until they are able to plant and harvest once again. Why are those who observe shemitah rewarded with such an unnatural occurrence?
The Midrash Tanchuma (Vayikra 1:1) comments:
"The strong warriors (giborei ko'ach) who do Hashem's bidding, to obey the voice of His word (Tehillim 103:20)"...Rav Huna explained in the name of Reb Acha: this refers to Klal Yisrael who said na'aseh before nishma (and thus accepted to perform mitzvos before they heard what their obligation entailed). Reb Yitzchak Nafcha said: this refers to those who observe the laws of shemitah. And why are they called strong warriors? A person sees his field and his trees (being treated as if they are) ownerless, the fences are open, his fruits are being eaten, and he controls himself and does not protest. The Sages teach, "Who is a gibor - a strong person? One who controls his natural temperament. (Avos 4:1)"
The middah of gevurah involves controlling one's natural tendencies. One who overcomes feelings of anger or jealousy, or one who resists an improper desire for physical pleasure demonstrates inner strength. Such behavior is so fundamental to one's avodas Hashem that Rav Yosef Karo chose to begin his Shulchan Aruch (Orach Chaim 1:1) by alluding to this very idea. He writes, "One should be strong as a lion to rise up in the morning to serve his Creator" - echoing the statement of Reb Yehuda ben Teima (Avos 5:20) that one should be "strong as a lion to do the will of your Father in heaven." One who fights his natural inclination to continue sleeping is as powerful as a strong lion.
The Avos d'Rebbi Nosson (23:1) adds, "There is no one stronger than giborei Torah - the mighty warriors of Torah." Those who study Torah diligently also demonstrate the middah of gevurah. By sacrificing their time, their sleep, and even their financial well-being, they act against human nature. Similarly, the Gemara (Gittin 36b) says, "Those who are shamed but do not embarrass in return, they are insulted but do not respond...about them the posuk says, 'And those who love Him (Hashem) will be like the sun rising in its full strength.' (Shoftim 5:31)" One who resists his desire for retribution is compared to the strength of a rising sun.
When Klal Yisrael said na'aseh v'nishma they were acting against their natural instincts because most people would refuse to accept an undefined obligation. That is why Klal Yisrael are called giborei ko'ach because their statement was an expression of inner strength. And by the same token, those who observe shemitah also demonstrate gevurah by submitting their will to the command of the Torah to allow others to enter their fields during the year of shemitah and take fruits for free.
What is the reward for those who overcome their natural tendencies in order to serve Hashem? They are treated to extraordinary blessing. Chazal comment (Midrash Rabba, Vayeishev 87:8), "The sea split in the merit of Yosef's bones - 'The sea saw and fled (Tehillim 114:3)' in the merit of the one about whom it says, 'And he fled and went outside. (Vayeishev 39:12)'" Hashem performed a miracle and split the sea before Klal Yisrael in the merit of Yosef who acted with superhuman strength when he resisted the wife of Potiphar and he ran outside. One who acts against his nature is repaid with supernatural blessing. Similarly, Chazal say, "One who is not so exacting in the way he deals with others will have his sins forgiven. (Yoma 23a)" If someone overlooks a wrong that he has suffered, then Hashem will also overlook that person's wrongdoings. One who rises above his natural instincts when dealing with others is not judged in heaven by the normal rules.
Perhaps this is why those who observe shemitah are blessed with extra produce during the sixth year. By giving free access to their fields they overcome their natural impulse to act as owners. In return, Hashem promises that in the sixth year the fields will produce much more than their normal yield. The unnatural self-control of those who observe shemitah is rewarded with unusual blessing.
This idea - the importance of controlling one's natural impulses - is especially relevant during the period of Sefiras HaOmer. On Pesach we offer the Korban HaOmer which is made of barley, an animal food (see Sotah 14a), while on Shavuos we bring the Shtei HaLechem (two loaves) which are made of wheat, a human food. These korbanos symbolize the inner transformation that the Torah demands of us - to overcome our natural animal instincts for self-satisfaction, and to act instead with restraint and self-control. By observing mitzvos with dedication and discipline, and by interacting with others with humility and self-control, we demonstrate a powerful inner strength, and we make ourselves worthy of extra blessing.