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The German 6/5 chord, containing the augmented 6th, is a good tool for modulation. I'm not sure if this is what you're talking about but I'll post some information anyways:
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The German 6/5 is equivalent to a dominant 7th in sound. That is, by simply playing a dominant 7th or German 6/5 at the keyboard with no notational reference and without resolving either chord, the chords sound identical. The resolution determines the identity of the chord, and the resolution determines whether the chord is spelled as a dominant 7th or a German 6/5. The note determining the functional identity of the chord is the seventh of the dominant 7th and the "root" of the German 6/5 (the note that usually appears in the soprano and forms the interval of the augmented sixth with the bass).
Enharmonically, the note is the same in each chord. For example, in C major the German 6/5 is Ab-C-Eb-F#. The F# moves up to G in the resolution. If, however, we respell the F# as Gb, we generate a dominant 7th, Ab-C-Eb-Gb, that moves to a Db chord. How would a Db chord function within the context of C major? Answering this question highlights another interesting use of the Dominant 7th/German 6/5 duality; a German 6/5 that resolves as a dominant 7th becomes V of the Neapolitan. The German 6/5 must be the German 6/5 of the dominant of the key for this to work. One extended use of augmented sixth chords is to introduce an augmented sixth chord that moves to a chord other than the dominant. Since the German 6/5 has a dual nature, it is an excellent agent of modulation.
Composers often took advantage of the Dominant 7th/German 6/5 duality. One of the best examples I know is the opening of the second movement of Beethoven's Piano Sonata Op. 53, the Waldstein.
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