The interaction of the antiferroelectric helix with an electric field applied perpendicular to its axis is investigated, A two stage switching process is observed, in which the helix unwinds to an intermediate state in the pretransitional regime, at voltages below the threshold for antiferroelectric to ferroelectric switching. A series of experimental studies and theoretical modelling show that the intermediate state is an antiferroelectric state in which the directors lie in the plane containing the applied electric field and the smectic laver normal. The mechanism for the unwinding process is the interaction of the applied field with an induced polarisation, which arises from a distortion to the antiferroelectric ordering.