(notably at mm. 76 and 81) just before the transition to Section B. The Cross-Eyed Pianist Section B: B Minor (The "Trio")
On paper, E-flat major and B minor are distantly related. However, Schubert treats them as enharmonic neighbors. He uses G-flat (the 3rd of E-flat minor) as a pivot to F-sharp , which becomes the dominant (V) of B minor. schubert impromptu op 90 no 2 harmonic analysis