A dominant 7 and a German augmented sixth are the same four notes spelled differently. Play your 5⁷, hear it as the new key's Ger⁺⁶, and it resolves a half step below where everyone expected. Schubert's favorite trapdoor.
hear it in Schubert, Valses Sentimentales — the documented trapdoor
Lead-in — how players set it upArrive at 5⁷ by the most ordinary road available.Or voice the 5⁷ with the b7 on top — the respelled note leads.Or feint the honest resolution once before the trapdoor run.Follow — where it usually goesResolve the respelled chord to the new 5⁷, then home — a half step below the old promise.Furnish the low room — new 4 and new 6- make it real.Or respell back — the trapdoor swings both ways, a half step up.
The German chord's notes respell, and so can your melody: hold the b6, re-hear it as ♯5, resolve up — enharmonic singing.Target tones (in C)G7 — lean on B (3) · F (b7)F#7 — lean on A# (3) · E (b7)B — lean on D# (3)Half-step wiresG ↘ F# · B ↘ A# · A# → B · E ↘ D#Across the borderno shared tonic tones with B — cut clean and re-land