The mechanisms of nonlinear interaction of external helical fields with a rotating plasma are investigated analyzing the results of recent systematic experiments on the Joint European Torus (JET) [A. Gibson , Phys. Plasmas 5, 1839 (1998)] that widen the previous data base collected on Compass-D [T. C. Hender , Nucl. Fusion 32, 2091 (1992)], Doublet III-D [R. J. La Haye , Nucl. Fusion 32, 2119 (1992)] 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 mechanisms of plasma rotation braking, and spin up, associated with error fields are analyzed in detail and interpreted. (C) 2002 American Institute of Physics.