Everyone has 4 introverted function that generate light or more aptly “energy”. What is this energy exactly? Who knows. If you think of it as psychic stimuli, which are absorbed and processed by the extraverted functions, then you can model external reality too as a collection of discrete packets of these psychic stimuli.
Thus you can see that reality itself is also producing the same “energy” as the introverted functions, and in fact raw contact between the psyche and reality would be akin to grabbing a live wire. The ego is an “electrical insulator” for the rest of the psyche.
The ego is not an overseer, because that implies an active supervisory role. More aptly, it shades the unconscious mind from the “light” of reality. The ego and the shadow are codependent, sharing a symbiotic relationship, like between the cell and the mitochondria. The mitochondria generates power for the cell, and the cell protects the fragile mitochondria from the dangers of the external world.
Also see
Transclude of The-structure-of-the-human-psyche