We report experimental studies and suggest a quantitative model of spin relaxation in Mn-12 acetate in a pulsed magnetic field in the temperature range 1.9-5.0 K. When the field applied along the anisotropy axis is swept at 140 T/s through a nonmagnetized Mn-12 acetate sample, the sample's magnetization switches, within a few milliseconds, from zero to saturation at a well-defined field whose value depends on temperature but is quantized in units of 0.46 T. A quantitative explanation of the effect is given in terms of a spin-phonon avalanche combined with thermally assisted resonant spin tunneling. [S0163-1829(99)11529-7].