A 2D slice through the wind-collimated jet, illustrating the MKO model. The relativistic flow velocity, β, at a particular point in the jet is parallel to the local wind velocity, β_w. The wind propagates semi-ballistically, so different distances along the jet have local wind velocity directions out of phase. Since it is the aberration caused by the changing jet velocity that modulates the observed flux, frequencies originating from different locations in the jet that are observed simultaneously are at different phases in their sinusoidal flux variations.