This piece is called Soundings, because the outer sections of it have to do with the natural resonance of the piano - holding down some notes while playing others, etc.  The middle section is just a romp, an impromptu that is called “selah” in the score, because that’s what the Psalmist says to the musicians periodically, and it is thought to mean “play on!”  Performing this on a grand piano will produce much greater resonance in the outer sections of Soundings.

For video of performance, please see https://vimeo.com/275122388