  Based on the spin-blockade model for organic magnetoresistance we present an
analytic expression for the polaron-bipolaron transition rate, taking into
account the effective nuclear fields on the sites. We reveal the physics
producing qualitatively different magnetoconductance line shapes as well as the
ultrasmall magnetic field effect, and we study the role of the ratio between
the intersite hopping rate and the typical magnitude of the nuclear fields. Our
findings are in agreement with recent experiments and numerical simulations.
