When a large stars core finally collapses into a neutron star the event takes place in an instant. The core collapses and the outer layers of the star which were held in place due to the core now are pulled by gravity down where they will hit the core and bounce back up. This bouncing of the outer layers off the newly formed massive core are what cause a supernova. This process has only recently been able to be described in computer models by scientists.
(The Great Courses – The Life and Death of Stars)