a) the star of at least 9 solar masses becomes layered by element, with an Fe core
b) reaches the Chandrasekhar-mass and begins to collapse
c) degeneracy density causes a rebound in the core collapse
d) this rebound creates an outward shock front
e) the shock front is then stalled at a radius on the order of 100km
f) an uknown mechanism causes the stalled shock to continue outwards, leaving a proto-neutron star