  We consider the evolution of cosmological perturbations at the QCD
transition, in particular the sudden reheating from a supercooled universe to
the transition temperature. Sudden reheating happens at a specific temperature,
hence density, and singles out one specific hypersurface. Underdensities reach
the reheating earlier than overdensities, there is a short period of huge
pressure differences which leads to a jump in the fluid velocity. Density
perturbations of scales far below the Hubble radius $\lambda\ll R_H $ get an
amplification which grows quadratically in wavenumber. Primordial black hole
formation will not be sufficiently amplified by the QCD transition unless the
initial spectrum is fine tuned.
