Aura

"Power has become so subtle and complex a thing in the ways taken by the Ekumen that only a subtle mind can watch it work...one feels the man's power as an augmentation of his character"

I agree with the idea that the author has introduced with this statement. The way I see it, she is talking about a person's presence or aura which is a difficult thing to sense but is extremely powerful. Even if someone does not have much tangible power, this subtler aura someone gives off can easily manipulate people. In many ways this power over people is much more useful than tangible power itself.