Star wants to get rid of electrons. The neutrinos leave, and we have only neutrons left. Now there is no pressure at all opposing the falling core. Eventually, the density gets so high, the neutrons have a significant degeneracy pressure. The core stops falling with a giant jolt, called a bounce.
Heat generates zillions of neutrinos.
Density is so enormous, even neutrinos can't escape immediately. It takes ten seconds for energy of core to be released as neutrinos. During those ten seconds, the supernova is releasing more energy than the entire rest of the visible universe. Because neutrinos are hard to detect, we hardly notice. Shock wave leaving the core blows the rest of the star to pieces Shock wave gets reinvigorated because neutrinos dump about 0.1 percent of their energy into the wave.