On 12 August 2001 the Cluster spacecraft measured a rapid flux transport event, consisting of a strong perpendicular earthward flow burst combined with a dipolarization of the magnetic field after a strong compression of the magnetotail. Combining the Cluster data with those from ground-based magnetometers, we find that this event is related to patchy reconnection taking place in the tail. After the event the magnetotail is locally evacuated of magnetic field, and an increased plasma pressure takes over from the magnetic pressure. This situation lasts for 15 min, after which a new equilibrium is sought, resulting in an oscillating magnetic field with a period of 20 min. The rapid flux transport observed with Bz and vx is shown to be in agreement with Bx variation using Maxwell's equations. The oscillation period agrees well with what is theoretically predicted. Our results show how a damped eigenoscillation of the magnetotail can be initiated by fast flows.