22.41Tritone substitution creates chromatic ii–V bass

Normal:

Dm7 → G7 → C

roots:

D → G → C.

Substitute:

Dm7 → D♭7 → C

roots:

D → D♭ → C.

The new bass line is almost too perfect:

whole progression descends chromatically.

This is why the substitution is so common.