Answer: Machsom is a transition from one quality to another, when a person changes his qualities from egoistic to altruistic, from the ability to work only for oneself to actions for the sake of others or for the sake of the Creator. In other words, his quality of giving prevails over the quality of receiving.
Question: Which of the Kabbalists first used this term?
Answer: I first heard it from my teacher Rabash. And then this term comes up several times in our sources.
Question: Did Baal HaSulam write that this is the psychological boundary between the two worlds?
Answer: This is not just psychology, but a sense of your attitude to the world, a change in human nature.
Comment: It is difficult to understand how you can do an action without expecting some benefit for yourself. Even today, psychologists say that this is impossible.
My Response: Indeed, it is impossible. But the fact is that the correct utilization of the forces of nature—when a person, in addition to the power of receiving, begins to awaken the quality of bestowal—gives him the opportunity to own both forces and use them together.
Or, at first, he goes only to the force of bestowal, which is called the quality of bestowal or faith (Emunah in Hebrew), and then he can act with receiving for the sake of bestowal.
From KabTV’s “Spiritual States” 2/6/20