The matter of the star exerts a downward pressure on the electric-interacting particles. The neutrinos at first are just held in by the random-walk they do from colisions in the very dense part of the star. The trigger for the release is neutrinos leaving the super-dense-random-walk area, allowing it to cool down crazy fast. So when they neutrinos start to emerge they leave the core of the star at near light speed, the same way photons leave the edge of the star. IIRC, they do not need to be accelerated from collapsing to expanding, but the mass does. (took physics an eternity ago, so do not quote me)
The matter of the star exerts a downward pressure on the electric-interacting particles. The neutrinos at first are just held in by the random-walk they do from colisions in the very dense part of the star. The trigger for the release is neutrinos leaving the super-dense-random-walk area, allowing it to cool down crazy fast. So when they neutrinos start to emerge they leave the core of the star at near light speed, the same way photons leave the edge of the star. IIRC, they do not need to be accelerated from collapsing to expanding, but the mass does. (took physics an eternity ago, so do not quote me)