For example, when traveling to the IV chord, some progressions may temporarily treat the IV as a I and travel to it using a ii-V-I progression within that temporary key. In the key of C, this would consist of using a Gm7 – C7 – F progression to travel to a F chord. There is a Bb within the Gm7 chord and the C7 chord. This pitch does not exist in the key of C, but is utilized as a temporary excursion while traveling to the F chord (as if we are moving to the key of F).
Another example is used when traveling to the vi chord. If we temporarily treat vi as i we can use a minor ii-V-i progression to travel to the vi chord. In minor ii-V-i progressions, the ii chord is a minor seven flat five chord. In the key of C, this would consist of using a Bm7b5 – E7 – Am7 progression to travel to an Am7 chord. The E7 chord contains a G# which does not exist in the key of C, but is utilized as a temporary excursion while traveling to the Am7 chord.
The concept of triton substitution allows us to further modify the two examples listed above. The third and seventh of a dominant seventh chord are the interval of a triton. Since a triton is the symmetrical bisect of an octave, those pitches will also be the seventh and third of the dominant seventh chord that is a triton away. For example, the third of a C7 chord is E and the seventh is Bb. If we travel up a triton from C to Gb, the third of a Gb7 chord is Bb and the seventh is Fb (or E).
This shared third and seventh relationship between dominant seventh chords that are a triton apart causes them to be interchangeable harmonically. Therefore, in the examples above, the dominant seventh chords can be interchanged for their triton substitution chords. When we are traveling to F (in the key of C) this would result in a Gm7 – Gb7 – F progression. When we are traveling to Am7 (in the key of C) this would result in a Bm7b5 – Bb7 – Am7 progression. Notice that this option allows for a chromatically descending bass line.
These are just some of the chromatic excursions possible due to the circle of fifths. Trying to write about every possibility in every musical style would result in a book as opposed to a blog entry. Understanding the examples listed above will give you the understanding to be able to explore and discover additional harmonic options.