Twisting rate of mean twist over the main FR field lines (black curve), and the photospheric vorticity twisting proxy ω at the negative-polarity footpoints (red curve). Both quantities have been smoothed temporally using a boxcar of 36 min to remove small-scale oscillations and to better bring out the trends. Vertical dashed line shows the time of the eruption on 22 April 2013 at 08:40 UT.