Simulation’s time evolution of the field-line map in a y − z plane through the middle of the sheared arcade during eruption onset. The z = 0 surface is the magnetogram of Fig. 1a. Panels a and b show the direction of the Lorentz force (red arrows), which is favourable for driving the anti-parallel field lines into close proximity and developing an X-type geometry. Panels c and d are superimposed with log Q, confirming the enhancement of Q-values co-located with the X-type geometry. Panels e and f are overplotted with |J|/|B|, which also shows an overall increase in the |J|/|B| in the vicinity of the X-type geometry (panel f).