Modal Jam Theory
When the plain symmetry of the tritone interval is intentionally avoided... the spiral symmetry of the golden ratio (and its reciprocal) is revealed as the catalyst for modulation.
The Tri-tone Substitute
Modal Jam Theory is anything but conventional (actually - it's much more 'stream-lined' than what is described in this article) - but for a better understanding of how the 'Circle Of 'C' can be divided by the 'G' and the 'Db' -- we must explore what can be found in most conventional textbooks: The Tri-tone Substitute.
The short story of everything to follow in this article is that the 'Db7' produces the same critical 'active' tones ('Fa' and 'Ti' in the key of 'C') as the 'G7.' Both of these chords may set the ear's strong anticipation to resolve to 'C.'