When the Or HaChaim HaKadosh zy'a lived in Morocco, he changed the workweek schedule. He said, "The week has six workdays. It's sufficient to work on Sunday, Monday, and Tuesday. Leave Wednesday, Thursday, and Friday for Torah. I promise your parnassah won't suffer because of it." His community followed his advice; indeed, they had parnassah just like before when they worked all week long. They discovered they could spend much time learning Torah each week, and Hashem would support them. This went on for many years.
When the Or HaChaim moved to Eretz Yisrael, the Moroccan community gradually began to return to their past habits of working more days. Eventually, they were back to a six-day workweek. But they admitted that the extra working days didn't bring them greater financial success. They were just as well off when they followed the Or HaChaim's counsel הוי בתורה ועסוק בעסק מממעט, to work less and to study Torah. They realized that parnassah is from Hashem, and Hashem can support them when they work less, too.
This is the lesson of shemitah. To know that Hashem provides support, and parnassah comes from Him.
