The outer core of this piece is firmly rooted in the Lochrian mode (all the white notes from B-B on the piano) with no chromatic elements present from 1-14 and 48-73. By contrast, its central portion is tonally ambivalent, the music forever changing its centre of gravity. Though it is the Lochrian mode that, above all, gives the piece its essential flavour, there is a subtle change to the Phrygian (white notes E-E) near the end (66-69), resolving unambiguously to A minor in the last 4 bars.
The listener must be able to "feel" the slow rhumba rhythm from beginning to end, the tempo remaining constant throughout. The performance should be very relaxed and unhurried and nowhere should the dynamic be more than a mezzoforte.
Difficulty level, roughly compared to ABRSM exam grades. 0 is beginner, 9 is advanced (beyond grade 8).