This work is by far the closest to the bone piece of piano music I can imagine. Technically it is beyond difficult, calling for a range of dynamics, tempo, power, touch, and everything else that might affect the sound of an instrument too complex to describe or notate.
Wallace Stevens got it right:
"It is a thing to have,
A lion, an ox in his breast,
To feel it breathing there. ..."
Playing it went far beyond any experience I have ever had except, perhaps, as a child hearing for the first time a sound that came like a lion directly into my soul.