Heat leaks upward out of the molten core faster than usual through this cold finger. Light, nonmetallic elements like sulfur are released at this boundary. This light material rises, stirring the liquid outer core in the process. Responding to Earth's rotation, it swirls in a spiral up toward the rocky mantle. Flowing electrons in the molten nickel and iron produce an electric current.
Earth rotates the electric current, making a dynamo and thereby producing a magnetic field.
Cold rock replacing hot rock at the core causes a little of the liquid outer core to solidify, setting off a train of events that result in a magnetic field. Maybe just a little bit. Such reversals happen every few hundred thousand years or so. This ionizing radiation damages the DNA of all living things. We are getting far from the cause here, but the thread still holds.