How does a ferrite bead or core reduce common-mode RF current on the shield of a coaxial cable?

A ferrite bead or core placed around a coaxial cable acts as an RF choke. The magnetic properties of the ferrite material add inductance to the outside of the coax shield, which creates a high resistance (specifically, an impedance) to radio frequency currents trying to travel along it. This added impedance effectively chokes off unwanted "common-mode" currents—currents that flow on the outside of the shield and cause the cable itself to radiate or pick up RF interference. Ferrites do not create out-of-phase canceling currents, nor do they magically convert common-mode currents into desired differential-mode currents. They simply act as a roadblock (impedance) to stop the unwanted RF current from flowing.