Evolution of the rotating flux tube. Top: superimposed snapshots in IRIS FUV SJI data during three different phases of the torsional oscillation of the surge material. White dotted lines outline the helical trajectories of the plasma threads. The arrow shows the direction of the rotation. The white vertical line corresponds to the fraction of the slit intersecting the surge plasma. The freely moving and confined component of the ejected surge material are denoted S1 and S2, respectively. Bottom: IRIS Si IV and Mg II K spectra of the surge material corresponding to the fraction of the slit marked by the white line. We show spectral snapshots from three different phases of the motion of the plasma at the slit position. In each panel, the vertical dashed line indicates the mean Doppler velocity of the plasma. The alternating tilt of the emission in the different phases, denoted in each panel by the tilted dashed line (obtained by visual examination) corresponds to the torsional motion of the surge material. Animation of this figure is available online.