Here is a simplified picture of what happens in a Type II.
The star's structure is supported against gravity by the heat being generated in its core by fusion processes. When the star burns all the fuel in its core to iron, fusion stops (the heat source is extinguished) and gravity then takes over and collapses the star.
The core of the star then gets tremendously squeezed down by the huge pressure applied to it by the inrushing mass of all its outer layers. It resists that pressure and rebounds, kicking the inrushing matter back out again with tremendous velocity as a shock wave traveling outwards against the inrushing matter. When the shock wave makes it all the way out to the surface of the star, the outer layers and a lot of the iron core get blasted out into space.
During the crushing of the core, the pressure is great enough to mash electrons onto the protons in the iron nuclei, which releases a gigantic flood of neutrinos. Moments later, those neutrinos emerge from the exploding star and zoom off into space.
Hours later, when the shock wave makes it out, a huge burst of ultraviolet and visible light comes streaming out of the remains of the star.
In the weeks and months that follow, the highly radioactive nickel and cobalt nuclei that were formed out of the iron in the core decay and spewed out into space release radiation, the energy of which is a telltale sign of that decay.
The supernova of 1987 provided us with the early neutrino burst, the ultraviolet burst, and then the nickel-cobalt-iron decay radiation, confirming this picture of the Type II supernova mechanism.