[Suzerain of Sheol]

Well, I imagine that whatever effect the psychic is trying to achieve meets some sort of "resistance," just for most day-to-day stuff said resistance is negligible enough not to mention. The more powerful an effect would be, the more draining it'll be on the psychic's brain. Which is at least part of the reason why Emim are so incredibly dangerous -- they're basically incarnate thought and can overcome nearly any resistance, since they'll never tire.