Schubert’s Impromptu D.935, Op. posth. 142 – No. 2 is one of my favorite piano pieces. It has many delights, but what really caught my ear was the bombastic double forte section (m. 17-30 at 1:06). It takes us into the IV key and uses a clever trick to modulate back so that I barely noticed when we arrived back home.

Below are lead sheet-style chords under their functions, with simple inversion notation. I’ve transposed from Ab to G to get rid of the double-flats (I’m terrible playing/thinking in Ab).

17
G:   I:1           I              vi   V7:1   V7             I
     G/B    |      G       |      Em   D7/F#  D7      |      G

21
G:    IV
C:    I                                iv:1   iv             I
      C     |              |           Fm/Ab  Fm      |      C

25
C:   iv:1        Ger+6           i:1
G:                               iv:1                      Ger+6
    Fm/Ab   |     Ab7      |     Cm/Eb                |     Eb7

29
G:   I:2          V7
     G/D    |     D7

Measures 17-20 are a simple diatonic I vi V7 I. M. 21-24 move us to the IV key and borrow Fm from its parallel C minor.

The magic for me is in m. 25-28: Schubert uses the fact that the 1st inversion borrowed iv chord (Fm/Ab in C) is only one note away from the augmented 6 chord (Ab-C-F# AKA “Ab7”), so he raises the F to move between them. But there’s more: While we were in C major moments ago, he treats m. 25-27 as in C minor and follows the +6 with the minor tonic, Cm. After all, he did foreshadow C minor by borrowing Fm. But that Cm, like the Fm, is also in 1st inversion, so he again raises one note (C to C#) to get the +6 chord in G, providing a transition back to the home key.

In summary he uses the same iv:1 -> +6 -> tonic trick twice in row, each time raising a tone to an unexpected accidental while moving the tonic down a fourth. While neither the iv:1 nor the +6 chord are super mysterious by themselves, I find this movement disorienting in a wonderful way. Out of that cloud you land in a warm cadential 6/4 at m. 29.

The following section also has a nice darker cousin to the cadences we heard earlier (m. 35 at 1:47):

35
G:   I7:3        ii:2 V7/ii       ii°:2  I:2 V7   I
     G7/F      | Am/E  E7      | Adim/Eb G/D D7 | G

It keeps getting darker by flattening more notes: First the 7th (F# to F), then the 2nd (A to G#), and finally the 6th (E to Eb) by borrowing the diminished ii from G minor.