At the beginning of Tanya chapter 29, the Alter Rebbe quotes the statement of the Zohar: If the wood doesn’t catch on fire, we break it (up into smaller pieces). Similarly, if the body doesn’t become lit with the light of the Neshama, we break it. The Alter Rebbe then explains that by reflecting upon a meditation that makes the body feel low, we make it receptive for the light of the Neshama.
This is also the idea of iskafya, self-restraint.
Men darf zich a beig ton. We have to restrain ourselves from indulgence.
But what does this involve? Starving ourselves? Of course not—we must be healthy in order to serve Hashem. Rather, it means regularly holding ourselves back from some material activity for which we have a strong desire. It means that when you have a strong desire for some material pleasure, you davka don’t give in to it. And even if you don’t hold yourself back every time, at least make it a regular part of your behavior.
Iskafya depends upon the individual. For many, perhaps most, it means indulging less. However, it could mean davka eating for someone who doesn’t want to eat. When Rabbi Gershon Mendel Garelik, Rav of Italy, was a young student, the Rebbe told him in Yechidus to make sure to finish everything on his plate. The reason seems to have been that he ate very little and was of ill health. Similarly, each person has to assess individually in which areas he needs to break himself.
In any case, the purpose isn’t to break and deprive the person for its own sake, but to change one’s inner self. Every single time the person breaks himself, he becomes more aidel, refined, and more of a keli, more receptive. More easily able to grasp abstract concepts in Chassidus. More easily concentrate on meditation in Tefillah and the words of Tefillah. More enthusiastically and genuinely do a favor for a fellow Jew. Try it and see.