According to jazz theory, we can substitute a dominant seventh chord with another dominant seventh chord that shares the same tritone. Both chords are shown here. They both contain the tritone A-Eb/D♯. The chords are also a tritone apart from each other (see Tritone Substitution for more information):

Here is the V7/V-VI in A minor:

We changed the first chord using tritone substitution...

By enharmonically changing the E-flat to a D-sharp, we create an augmented sixth chord:

The tritone substitution and the augmented sixth chord both explain similar concepts.