It is shown that the large reservoir of thermal energy inside the neutrino-sphere left behind by the shock formed in stellar collapse cannot be tapped on shock time scales by either mu, tau, or electron-type neutrino diffusion. Hence, the shock cannot be strengthened or energized by neutrino transport. It is also shown that after the shock breaks out of the neutrino-sphere, there is a significant and unavoidable decrease in electron fraction due to rapid electron capture in this quasi-transparent region, despite the kinetic equilibrium that is set up between electron capture and electron-neutrino absorption. A trough in electron fraction must form in a few milliseconds and deepen. The subsequent pressure deficit (10-35%) and rarefaction are likely to play a significant role in stopping the shocks of current hydrodynamic calculations. The dependence of the time history and profile of the electron fraction on shock speed, the magnitude and decay time of the neutrino flux, and the matter density profile are investigated.