It is perplexing, for since there is neither Rosh nor Sof there, how is there middle? Moreover, are we dealing with a corporeal matter here? (Baal Hasulam, Study of the Ten Sefirot, Vol.1, Part 1, Chapter 1)
Naturally, we are not talking about anything corporeal. We are only talking about qualities.
On the one hand, it is contained by the Creator, who nourishes, supports, and develops it, yet it is absolutely opposite to Him. Otherwise, it would not exist.
Creation must exist, as if, within the Creator, in His qualities, means, and forces; on the other hand, it is absolutely opposite to Him, otherwise it would not be a creation, something that is separated from the Creator. This is how it develops.
Question: Why did the restriction occur at the central point? It turns out to be some kind of geometry.
Answer: Here it means that all the intentions, all the actions of the Creator, all His thoughts and aspirations are directed to this central area, which He has designated in this way, and to the will to receive, which develops in this area.
From KabTV’s “The Study of the Ten Sefirot (TES)” 9/4/22