Top: radial profile of the plasma bulk velocity components V/c of the striped wind averaged in φ (solid lines) and the analytical E × B drift velocity of a pure monopole (dashed lines) at t = 25.76P in run R3_S250_K10. Bottom: corresponding φ-averaged bulk Lorentz factor of the striped wind (blue line). The wind first accelerates approximatively as (dashed line) and then grows as (dot-dashed line) passed the fast point (yellow star) where the (red line) and the Γ curves intersect.