Q: Like if you playing a blues song in C and for the little V-IV-I turnaround you hit a G#7 before going to the G7... would that be a tritone substitution too (the 5th of G is D, tritone of D is G#)?
A: i think tritone substitution refers to any time you have a 7th chord with the tritone harmony in it (like a G7 chord contains the notes B and F) and then you switch the bass note for a Db note, which essentially makes a Db7 chord instead, because the core notes (B, F) are the same. Additionally, any other notes you were using in the chord can usually translate to the other, and usually a light-dark or dark-light transformation occurs. for example if you had G7 with 9, 13, (the 'light' notes) and you flip the bass to Db, and leave all the same notes, they become the 'dark' notes on the Db7 so it becomes Dbaug7#9. This whole line of thinking is generally only applicable in the school of thinking that the notes B and F with G in the bass should resolve to C and E, with C in the bass. or B and F with Db in the bass resolve to F# and A# with F# in the bass. so why not switch