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.