  We formulate and quantify the spin-orbit torque (SOT) in intrinsic
antiferromagnetic topological insulator $\rm MnBi_2Te_4$ of a few
septuple-layer thick, which exhibits conspicuous layer-resolved
characteristics. Contrary to known current-induced torques, the SOT in
insulating $\rm MnBi_2Te_4$ is driven by an electric field (or voltage). We
further study the SOT-induced magnetic resonances, where in the
tri-septuple-layer case we identify a peculiar exchange mode that is blind to
microwaves but can be exclusively driven by the predicted SOT. As an inverse
effect of the SOT, topological charge pumping generates an adiabatic current
devoid of Joule heating, which occurs concomitantly with the SOT and gives rise
to an overall magneto-reactance for $\rm MnBi_2Te_4$, enabling a lossless
conversion of electric power into magnetic dynamics.
