The mechanisms of nonlinear interaction of external helical fields with a rotating plasma are investigated analyzing the results of recent systematic experiments on JET that widen the previous data base collected on Compass-D, DIII-D and JET. The empirical scaling laws governing the onset of "error field" locked modes are re-assessed and interpreted in terms of existing driven reconnection theories and with new models. In particular the important mechanism of plasma rotation braking and spin up associated with error fields are analyzed in detail and interpreted.