There is evidence from different tokamaks that the onset of locked modes driven by resonant helical error fields or linked to spontaneous MHD activity [1], produces a dramatic bulk braking of plasma toroidal velocity that leads to a self-similar quench of the whole velocity profile on short time scales. These observations are in contrast with the expected effect of an electrodynamic torque resonant at a rational surface r=rq We present a new theoretical model based on the toroidal viscous effects due to breaking of axi-symmetry caused by resonant and non-resonant helical field perturbations with (m=0,n) components. Analysis of the experimental rates of mode growth (reconnection rate) and rotation damping sheds light on the prevalent physical mechanisms.