Time-resolved small angle X-ray scattering has been used to study the electric field-induced motion of the smectic layers in an antiferroelectric liquid crystal on switching between the field-induced ferroelectric states. The intensity and position of the Bragg peak was recorded with 10 mu s time resolution across a bipolar switching pulse of period 2 ms. Reversible layer reorganization and motion were observed to occur within the first 500 mu s of applying the switching field. The data show that the antiferroelectric liquid crystal switches from one ferroelectric state to the other with an increase in layer spacing of around 0.1 angstrom, a rotation of the layer normal of 0.5 degrees and a reduction in the angular width of the diffraction peak by 1.7 degrees. Subsequent relaxation of the layers occurs with the characteristic of a van der Pol oscillator. A model is presented that illustrates the form of the rapid and reversible layer motion.