22.4Dominant substitution begins with the tritone

Take:

G7 = G B D F

The notes that really define its dominant tension are:

B and F

They form a tritone.

Now consider:

D♭7 = D♭ F A♭ C♭

C♭ sounds like B.

So D♭7 contains:

F and B

enharmonically.

Same tritone.

This is the key.