Jazz theory explains that we can substitute a dominant seventh chord with another dominant seventh chord that shares the same tritone. Both chords are shown here. They share the tritone A-Eb/D#. Also, the chords are at the distance of a tritone from one another (see Tritone Substitution for more information):

Here we have the V⁷/V - V - i in A minor:

we change the first chord using tritone substitution...

by enharmonically changing the Eb into a D# we get an augmented sixth chord:

Tritone substitution and the augmented sixth chord are two ways of explaining similar concepts.

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