  Despite recent progress, laminar-turbulent coexistence in transitional planar
wall-bounded shear flows is still not well understood. Contrasting with the
processes by which chaotic flow inside turbulent patches is sustained at the
local (minimal flow unit) scale, the mechanisms controlling the obliqueness of
laminar-turbulent interfaces typically observed all along the coexistence range
are still mysterious. An extension of Waleffe's approach [Phys. Fluids 9 (1997)
883--900] is used to show that, already at the local scale, drift flows
breaking the problem's spanwise symmetry are generated just by slightly
detuning the modes involved in the self-sustainment process. This opens
perspectives for theorizing the formation of laminar-turbulent patterns.
