  Neutron stars may be surrounded by fall-back disks formed from supernova
core-collapse. If the disk circumscribes the light-cylinder, the neutron star
will be an active radio pulsar spinning down under the propeller spin-down
torque applied by the disk as well as the usual magnetic dipole radiation
torque. Evolution across the P-P_dot diagram is very rapid when pulsar
spin-down is dominated by the propeller torque. This explains the distribution
of pulsars in the P-P_dot diagram.
