The effect was known as 'rudder reversal', but not necessarily associated with a fin stall.
Look at the hinge line of the rudders on the fin; in extremis it was possible to put in a 'bootful' of pedal which would move the rudder to full deflection. This would then put more airflow on to the forward part of the rudder assembly (ie forward of the hinge line), and effectively 'lock' it in place as the aerodynamic forces were greater than the foot force the pilot could apply through the pedals. This led to an almost instantaneous spiral dive with increasing speed requiring even more pedal force............... This was actually a potential problem with all twin tail heavies hence the redesign of the Halifax tail and the early Manchester/ Lancaster. However rudder design did a lot to mitigate said problem- on the Shack MOTU we were told about it but then told we could ignore it as it wouldn't happen to us (mind you we were told a lot of things couldn't happen to the aircraft which did!).
Of note a relative of my father was killed when his Halifax spiraled in with suspected rudder reversal whilst carrying out radar trials from Defford. Unfortunately there were also a number of radar boffins on board as well.