When the current is applied, it creates magnetic flux. This passes into the rotor portion of the field. The hysteresis disk physically passes through the rotor, without touching it. These disks have the ability to become magnetized depending upon the strength of the flux (this dissipates as flux is removed). This means, as the rotor rotates, magnetic drag between the rotor and the hysteresis disk takes place causing rotation. In a sense, the hysteresis disk is pulled after the rotor. Depending upon the output torque required, this pull eventually can match the input speed, giving a 100% lockup.