Watch the Shell Voicings online guitar lesson by Frank Potenza from Jazz Harmony Handbook
The tonic chord in "Circle Changes" is typically a major 6th or 7th chord, as opposed to the dominant 7th tonic chord seen in major blues progressions. The progression that follows in bars 2-4 is a cycle that eventually brings us to the IV chord in bar 5, which is again a major 6th or 7th chord instead of the dominant 7th we've seen in major blues.
In bar 6, we have a II/V (Bbm7/ Eb7) that takes us back to the F6 tonic chord. This is commonly referred to as the backdoor progression, an optional way of cycling back to the tonic. In bar 8, we see a chromatic II/V sequence, followed by a long II/V in bars 9-10 and a diatonic turnaround in bars 11-12.