Jazz theory explains that we can substitute a dominant 7th chord with another dominant 7th chord that shares the same tritone. Both chords shown here share the tritone A - Eb/D#. Also, the chords are at the distance of a tritone from one another:
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:
We can see that the tritone substitution and the augmented sixth chord are two ways of explaining similar concepts.