  Prompt gamma-ray burst (GRB) emission requires some mechanism to dissipate an
ultrarelativistic jet. Internal shocks or some form of electromagnetic
dissipation are candidate mechanisms. Any mechanism needs to answer basic
questions, such as what is the origin of variability, what radius does
dissipation occur at, and how does efficient prompt emission occur. These
mechanisms also need to be consistent with how ultrarelativistic jets form and
stay baryon pure despite turbulence and electromagnetic reconnection near the
compact object and despite stellar entrainment within the collapsar model. We
use the latest magnetohydrodynamical models of ultrarelativistic jets to
explore some of these questions in the context of electromagnetic dissipation
due to the slow collisional and fast collisionless reconnection mechanisms, as
often associated with Sweet-Parker and Petschek reconnection, respectively. For
a highly magnetized ultrarelativistic jet and typical collapsar parameters, we
find that significant electromagnetic dissipation may be avoided until it
proceeds catastrophically near the jet photosphere at large radii ($r\sim
10^{13}--10^{14}{\rm cm}$), by which the jet obtains a high Lorentz factor
($\gamma\sim 100--1000), has a luminosity of $L_j \sim 10^{50}--10^{51}\ergs$,
has observer variability timescales of order 1s (ranging from 0.001-10s),
achieves $\gamma\theta_j\sim 10--20 (for opening half-angle $\theta_j$) and so
is able to produce jet breaks, and has comparable energy available for both
prompt and afterglow emission. This reconnection switch mechanism allows for
highly efficient conversion of electromagnetic energy into prompt emission and
associates the observed prompt GRB pulse temporal structure with dissipation
timescales of some number of reconnecting current sheets embedded in the
jet.[abridged]
