This consists of bVIIx, followed by a return using the circle of 4ths, e.g:
bVIIx - VIx - ii.
bVIIx is a substitution for IIIx or iii.
The chords in the sequence may be used diatonically (e.g. bVIIx - vi - ii - V - I) or changed to dominants.
references:
Tangerine