Motion of a star ensemble around around a perfectly cylindrical toy-model source. The initial velocity is v_z = 90 km s⁻¹, with the same conventions as in Fig. 1. Upper plots: a stream forms along a spiral trajectory. From a helicoidal fit to the cloud of points, as needs to be done on real data, with χ²/N_gdl = 1.16 we find a value for the torsion of τ ≃ 1.1 × 10⁻² kpc⁻¹ (lower plot). This is in agreement with the analytical estimate in the main text below Eq. (16).