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tokens: cool prominence plasma being ejected away from the reconnection region. The bulk of the ejected material consists of two main components denoted as S1 and S2 in Fig. 2 . Component S1 moves upwards freely and undergoes helical motion believed to be the result of the reconnection of a twisted bundle of prominence threads with overlying field lines. Helical motion of the ejected surge material suggests that the reconnection lead to the release of the magnetic twist accumulated in the flux rope supporting the prominence. The ejection of component S2 is triggered by the reconnection of a prominence thread with a closed coronal loop. The confined material then moves along a closed loop-like trajectory and subsequently falls towards the solar surface. Fig. 2. 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 . The direction of the helical motion of the surge material alternates, suggesting a large-scale torsional wave is set up in the flux tube following the reconnection. The helical motion of the individual threads is observable both in the imaging and spectral data. Rotation in a clockwise direction viewed edge-on is revealed in spectral data as opposite red and blue-shifted emission on the top and bottom edges of the flux tube, relative to the mean line-of-sight velocity. The reverse is true for a counter-clockwise rotation. The IRIS SG slit is in sit-and-stare mode and crosses the surge material at an angle. As a result, it is possible to determine the presence and direction of rotation from variations of the Doppler shift as a function of distance along the slit. Figure 2 shows SJI snapshots at various phases of the torsional motion of the flux tube as well as the Si IV and Mg II K line profiles along the slit between y = −254′′ and y = −244′′, where the slit intersects with the surge material. We note that the imaging and spectral snapshots are not concurrent because by the time the third phase of the propagating oscillation is visible in the