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.