Then two things happen. Suddenly all the protons in the star's interior are able to gain energy by eating a nearby electron and spitting out a neutrino. In the process they become neutrons, and the inner part of the star becomes a neutron star. While this is going on the outer part of the star is being blown outward by the recoil energy of the bounce. That outflow of energy is now boosted by the additional energy carried to the shock wave by the flood of neutrinos from the core collapse.
Neutrinos are normally very passive particles, electrically neutral and with a mass at or near zero.
A neutrino can pass through a light year of lead without scattering from even one lead atoms. Neutrinos usually mind their own business and go their own way without much association with normal matter. But within the passage of a few seconds in the supernova every proton in the interior of the star gives birth to a neutrino. All of these neutrinos streaming out from the heart of the star must travel through an amount of matter equivalent to a few thousand light years of lead. If you were in a Jupiter-type orbit a billion kilometers from SN1987A when it exploded and were protected from the other effects of the supernova, you would be killed by the radiation damage from neutrinos streaming through your body.