we're content with simply calling a Mystery a Mystery
So are we…but that doesn't keep a guy from thinking about things.
I certainly don't know enough either academically or exeriencially about essence and energies to say anything definitive or new that you've not likely encountered before.
Conceptually it is rooted in the distinction between the created and the uncreated and how communion exists between the created and the uncreated. It is clear the uncreated can go over to/interact with the created but the created can only go so far towards the uncreated. The gulf is not bridgeable in the creature. Man simply cannot be united to God in His essence. That is not communicable
Yet we know God interacts with and even entered creation. We call this the energies of God…not like a mere emanation of physical energy that is separate from the thing itself…no mere star wars style "force"..rather the energy is God too, but not God in His essence but God in His revelation and interaction.
Simplistically speaking, if not pressed too far, I suppose we could use the analogy of the Sun. The light of the Sun is not other than the sun, but it is the sun as it communicates itself to the world. We can know and be enlivened by this light and warmth. But what we cannot know is the sun in its essence…directly…it would destroy us…reduces us to streaming plasma. What it is is communicated to us by its light and heat…via the light we have a connect to it's essence, but no coinherence with the heart of the sun directly is open to us.
With respect to the Holy Eucharist we are told it is Christ's Holy Body and Blood.
We understand Christ is fully God and fully man.
The holy have witnessed the Holy Gifts transfigured and made brilliant with the uncreated light at the consecration.
Others have seen it transform directly into human flesh…at least for a moment, sometimes longer.
The mechanism of how it all works I don't believe has been revealed, but the language of essence and energy exists as conceptual toolbox and safeguard about how we talk about and understand the nature of our communion with God in Christ. There is a sharing of His life. There is a cleaning and revelation of His image. There is a transformation into His likeness. There is a communion with Him, the Father, and the Spirit, and with the Church in the Spirit. We are made to be partakers of His nature by grace, but we are never made partakers of His nature by nature. We remain created tasters of the uncreated. He remains the uncreated sharer with creation.
That's the best I can do. Perhaps those with more patristic knowledge and resources can unpack this question more suitably.