Solo 2 introduces a bunch of new principles.
It starts off with a special chorus type riff; a stop chorus with some approach chords; the Db13 and the Gb9.
On the 2 bars that normally hold the IV chord F, we now see an F#dim7 chord show up.
A diminished or ‘dim’ chord functions as a passing chord or connecting chord from the IV chord to the I chord.
You can hear this passing function very clearly, when you play an F7 bar chord at the 8th fret.
When you add you middle finger to the fifth string, ninth fret you end up with an F#dim7 chord.
If you then go back to the C chord and play the bar chord at the 8th fret, you can see that the note on the fifth string goes up one fret again.
The dim7 chord is a stack of MINOR third intervals; 4 notes that are equally far apart.
Because of this we can move it around in steps of 3 frets (a b3 interval).
On the F#dim7 chord we can play it’s chord notes, which sounds really cool.
It is often referred to as a ‘dim run’.
Start the riff with your ring finger.