If you’re in C major, the V chord is G7, which contains a tritone between its 3 and 7, B and F. You can say, what if instead of B being the 3 of the chord and F being the 7, F is the 3 and B is the 7? Then you discover that the root of this other chord is C# (or Db). Because the function of a chord is largely dictated by the 3 and 7 in it, this means you can substitute any dominant 7 chord with another dominant 7 chord a tritone away and it will still basically function the same (this can create some extra dissonance before the cadence which is nice and it creates some nice chromatic movement in the bass).
skybrian|3 years ago
(He did say "with the flat seven" which sounded like another chord entirely, but that just means seventh chord.)