  Recent months have witnessed dramatic progress in our understanding of SGRBs.
There is now general agreement that SGRBs can produce directed outflows of
relativistic matter with a kinetic luminosity exceeding by many millions that
of AGN. The requirements of energy and compactness indicate that SGRB activity
can be ascribed to a modest fraction of a solar mass of gas accreting onto a
stellar mass BH or to a precursor stage leading inevitably to such an object.
Scenarios involving the birth of a rapidly rotating NS, or an accreting BH in a
merging binary driven by gravitational waves are reviewed, along with possible
alternatives (collisions or collapse of compact objects). If a BH lies at the
center of this activity, the fundamental pathways through which mass, angular
momentum and energy can flow around and away from it play a key role in
understanding how these prime movers can form collimated relativistic outflows.
Hypercritical flows near BHs, where photons cannot supply the cooling, but
neutrinos do so efficiently, are discussed in detail, and we believe that they
offer the best hope of understanding the central engine. On the other hand,
statistical investigations of SGRB niches provide valuable information on their
nature and evolutionary behavior. In addition, compelling evidence now points
to the continuous fueling of SGRB sources. We suggest that the observed late
flaring activity could be due to a secondary accretion episode induced by the
fall back of material stripped from a compact object during a merger or
collision. Important open questions are identified, along with the types of
observation that would discriminate among various models. SGRB jets may be one
of the few observable consequences of how flows near nuclear density behave
under the influence of strong gravitational fields. (abridged)
