Top panel: zoom of the J1038 2D spectrum around the [O III] line. Lower panels: non-parametric velocity measurements as a function of the position along the spatial axis from which 1D spectra have been extracted. Red and blue symbols refer to Hα and [O III]λ5007 line velocity measurements. Orange shaded areas refer to the regions associated with the knots in 2D spectra, as labelled in the top panel. In the first panel, V90 (positive) and V10 (negative) measurements are reported. The other panels show W80, V50 and Vpeak variations. For V10, V90, and W80, we show with solid lines the average velocities derived from the total integrated spectrum for the [O III] line (dot-dashed lines refer to ±1σ). For V50 and Vpeak panels, we indicate the 0 km s⁻¹ velocity with dashed lines. We refer to Sect. 4 for the description of individual non-parametric velocity estimators (and associated distinct kinematic properties).