An Aspect of Etzem - G-d's Essence

To understand this point in history, we need to step back and understand the total picture.Before the world was created, there was only Hashem.

Then, Hashem created a world, in order to have a "Dira be Tactoinm", (A dwelling place in the finite physical creation).

Chassidus goes on to explain that, Hashem wanted His unlimited Etzem to totally reside, in the completely, limited world.
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This is impossible according to the finite Seder Histalshilus, (which is based on cause and effect, and the "laws of nature"). It is the ultimate paradox, being the combination of two totally opposite things.

But beyond Seder Histalshilus, is Etzem, the Atzmus of Hashem, Who is totally beyond any limitation. Therefore, it can combine any paradox.

That is why it is referred to in Chassidus as Nimna Hanimnaos. Which generally means, the impossible of impossibilities. This is a revelation of Etzem in the world, (but Hashem is beyond even this.)
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An example of this emanation in the world, was in the Beis HaMikdosh, in the "Holy of Holies". Here is where the Aron Kodesh resided.

When you measured the Aron, it was 1.5 Amos wide; and the entire room was 10 Amos wide.

So when you measured from each side of the Aron, to the wall on either side, it should have measured 4.25 Amos.
The total of the two sides would be 8.5, add the Aron itself 1.5 Ama, and you have ten Amas, the total width of the room.

But that wasn’t the case. when you went back to measure the
Aron itself, it measured 1.5 Ama.
But when you measured from each side of the Aron, you got 5 Ama, for a total of ten Ama.

In other words, the Aron was there 1.5 Ama, but simultaneously it wasn’t there! This was a revelation of Etzem.
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How does Hashem achieve His goal, in the creating of the world ?

At Mount Sinai, the Jews heard the first two of the Commandments directly from Hashem, but their souls flew out of their bodies; because they could not bear to receive such a high revelation, and stay Neshamah Beguf (body and soul).

It was impossible, that a finite body, should be able to receive, an infinite revelation. So they asked Moshe to receive the rest of the commandments.

Because Moshe had a high soul, and was a Memutza Hamechaber, (an intermediary, that contains both aspects, of the two things he is uniting).

Therefore, he was able to receive the revelation of Hashem, body and soul together, and reveal the Torah into the finite world.
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So too now. Hashem achieves His goal, of revealing His Etzem into the world, through Moshiach. Therefore, Moshiach must be a Memutza Hamechaber.

Therefore, he contains the two things that he is uniting; an aspect of the Etzem of Hashem (Moshiach's soul), and a finite body. Moshiach is a microcosm, that reflects the macrocosm- the world.

When Moshiach's body totally assimilates the soul of Moshiach, coming from Etzem, then the world at large can assimilate the revelation coming from Etzem.
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Now we can understand, why the Rebbe said about the Previous Rebbe, that he is Atzmus Mahus, in a Guf Gasmi, (the Etzem of Hashem enclothed in a physical body); because this is the definition, of the soul of Moshiach.

The Rebbe explains (in Sefer HaSeichas, Nun-Beis, Parshas Bo, towards the end), that the assimilation of the soul of Moshiach, with the body of the Previous Rebbe, was not complete, it still was above the body.

Which is not true of this generation, when the process reaches its total synthesis, Neshamah Beguf.
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In order to be able for the world at large, to able to now receive the Etzem of Hashem, it must become a Keli, (a fitting vessel), for this infinite revelation.

This is why this generation is called "Ikvasa Demoshiacha", the heels of Moshiach. Because Etzem resides specifically the lowest levels. The upper worlds of angels, etc, only have a ray of Scheina.

Also, Etzem resides in as place, that has no Milas, no Tzir, no physical attributes; beyond the limitations of thinking and feeling. It resides in a place of total Bittel, like the heel.