  Transitional protostellar disks have inner cavities heavily depleted in dust
and gas, yet most show signs of ongoing accretion, often at rates comparable to
full disks. We show that recent constraints on the gas surface density in a few
well-studied disk cavities imply that the accretion speed is at least
transsonic. We propose that this is the natural result of accretion driven by
magnetized winds. Typical physical conditions of the gas inside such cavities
are estimated for plausible X-ray and FUV radiation fields. The gas is
molecular and predominantly neutral, with a dimensionless ambipolar parameter
in the right general range for wind solutions of the type developed by
K\"onigl, Wardle, and others. That is to say, the density of ions and electrons
is sufficient for moderately good coupling to the magnetic field, but not so
good that the magnetic flux need be dragged inward by the accreting neutrals.
