  Resonant field amplification or error field penetration involves driving a
weakly stable tearing perturbation in a rotating toroidal plasma. In this paper
it is shown that the locking characteristics for modes with finite real
frequencies $\omega_{r}$ are quite different from the conventional results. A
calculation of the tearing mode amplitude assuming modes with frequencies
$\pm\omega_{r}$ in the plasma frame shows that it is maximized when the
frequency of the stable backward propagating mode ($-\omega_{r}$) in the lab
frame is zero, i.e. when $v=\omega_{r}/k$. Even more importantly, the locking
torque is exactly zero at the mode phase velocity, with a pronounced peak at
just higher rotation, leading to a locked state with plasma velocity $v$ just
above the mode phase velocity in the lab frame. Real frequencies
$\pm\omega_{r}$, leading to a $v\rightarrow-v$ symmetry, are known to occur due
to the Glasser effect [A.H. Glasser, J.M. Greene, and J.M. Johnson, Phys.
Fluids {\bf 19}, 567 (1976).] for modes in the resistive-inertial (RI) regime.
This therefore leads to locking of the plasma velocity to just above the phase
velocity. It is also shown that similar real frequencies occur over a range of
parameters in the visco-resistive (VR) regime with pressure, and the locking
torque is similar to that in the RI regime. Real frequencies occur due to
diamagnetic effects in other tearing mode regimes and also show this effect,
but without the $v\rightarrow-v$ symmetry. Nonlinear effects on the mode
amplitude and torque for weakly stable modes or large error fields are
discussed. Also, the possibility of applying external fields of different
helicities to drive sheared flows in toroidal plasmas is discussed.
