Watch the Caravan online guitar lesson by Mimi Fox from Jazz Performance
The tri-tone substitution works because the tones in the substituted chord resolve well to the one chord. In this case, I am substituting a Gb7 for a C7chord. The notes in Gb7 are: Gb, Bb, Db, and Fb or E. The Gb resolves to the F of the one chord(Fminor)by a half step and the Db resolves to the C of theFminor chord (the 5th of the chord. These half step resolutions work really well and while half step resolutions aren't the only kind that work, they are arguably the strongest you can use.