The AM Herculis binaries (AM Hers) have high and low accretion states. A model is proposed in which the white dwarf’s rotation acts like an angular momentum storage battery for the binary orbit. The accreted material from the Roche lobe-overflowing secondary charges up this battery, whereas the magnetic torque of the white dwarf on the secondary discharges it back into the binary’s orbit. The Roche lobe’s volume becomes alternately larger and smaller than the secondary’s volume. This alternation causes the mass flow to vary between a high and low rate.