While the A section is dominated by triplets , the Trio introduces a waltz-like rhythm where the triplet often falls on beat two, adding a "stamping" accent.
Schubert modulates from E-flat major to its dominant, B-flat major , via a standard pivot chord (E-flat acting as IV in B-flat). This is classical procedure. But the second theme (bar 21) is where things get strange. schubert impromptu op 90 no 2 harmonic analysis
This is where the harmonic analysis becomes fascinating. The B section acts as a development, but it is driven almost entirely by sequential modulation. While the A section is dominated by triplets