  Adapting and extending a suggestion due to Page, we define a wormhole throat
to be a marginally anti-trapped surface, that is, a closed two-dimensional
spatial hypersurface such that one of the two future-directed null geodesic
congruences orthogonal to it is just beginning to diverge. Typically a dynamic
wormhole will possess two such throats, corresponding to the two orthogonal
null geodesic congruences, and these two throats will not coincide, (though
they do coalesce into a single throat in the static limit). The divergence
property of the null geodesics at the marginally anti-trapped surface
generalizes the ``flare-out'' condition for an arbitrary wormhole. We derive
theorems regarding violations of the null energy condition (NEC) at and near
these throats and find that, even for wormholes with arbitrary time-dependence,
the violation of the NEC is a generic property of wormhole throats. We also
discuss wormhole throats in the presence of fully antisymmetric torsion and
find that the energy condition violations cannot be dumped into the torsion
degrees of freedom. Finally by means of a concrete example we demonstrate that
even temporary suspension of energy-condition violations is incompatible with
the flare-out property of dynamic throats.
