  Important astrophysical sources, such as gamma-ray bursts (GRBs) or tidal
disruption events, are impulsive -- strongly varying with time. These outflows
are likely highly magnetized near the central source, but their interaction
with the external medium is not yet fully understood. Here I consider the
combined impulsive magnetic acceleration of an initially highly magnetized
shell of plasma and its deceleration by the external medium. I find four main
dynamical regimes, that (for a given outflow) depend on the external density.
(I) For small enough external densities the shell becomes kinetically dominated
before it is significantly decelerated, thus reverting to the familiar
unmagnetized "thin shell" case, which produces bright reverse shock emission
that peaks well after the prompt GRB. (II) For larger external densities the
shell remains highly magnetized and the reverse shock is strongly suppressed.
It eventually transfers most of its energy through pdV work to the shocked
external medium, whose afterglow emission peaks on a timescale similar to the
prompt GRB duration. (III) For even larger external densities there is no
initial impulsive acceleration phase. (IV) For the highest external densities
the flow remains Newtonian.
