  A model is developed for magnetic `propeller'-driven outflows which cause a
rapidly rotating magnetized star accreting from a disk to spin-down. Energy and
angular momentum lost by the star goes into expelling most of the accreting
disk matter. The theory gives an expression for the effective Alfven radius
$R_A$ (where the inflowing matter is effectively stopped) which depends on the
mass accretion rate, the star's mass and magnetic moment, and the star's
rotation rate. The model points to a mechanism for `jumps' between spin-down
and spin-up evolution and for the reverse transition, which are changes between
two possible equilibrium configurations of the system. In for example the
transistion from spin-down to spin-up states the Alfven radius $R_A$ decreases
from a value larger than the corotation radius to one which is smaller. In this
transistion the `propeller' goes from being ``on'' to being ``off.'' The ratio
of the spin-down to spin-up torque (or the ratio for the reverse change) in a
jump is shown to be of order unity.
