The dynamic response of a twisted nematic cell is explored using a convergent beam system and the fully-leaky guided-mode technique. Application of an ac field results in a realignment of the director on the time scale of a few milliseconds. Fitting the guided-mode reflectivity and transmissivity data taken at different times after removal of the ac field allows a complete test of the dynamic theory of the nematic director behaviour. Direct confirmation of the director back-flow is established during the first few milliseconds after removal of the voltage. The optical results when compared with predictions of model theory establishes fully the validity of the Ericksen-Leslie theory. Exploration of the time dependent guided-mode data taken as the director relaxes to the zero volt state shows how the various viscosity coefficients influence the director relaxation yielding values for these coefficients. (C) 2004 Elsevier B.V. All rights reserved.