How does a flap load relief system work?
I'm not sure how common flap load relief systems are - I know the Airbus A330 has them and I believe all the Airbus A3xx series do. I've also found at least one post that seems to imply it's common on most "large commercial" aircraft, which I assume encompasses most transport category aircraft.  

On the Airbus the system is based on airspeed - When the maximum flap extension speed (VFE) is exceeded by a certain threshold the ship's computer automatically retracts the flaps.
My understanding is the retraction is staged, so if you've got full flaps selected and exceed VFE for that flap setting the system takes out one notch -- presumably it would continue doing so if you're still exceeding the operating limitations of the flap system until it reaches a point where it's convinced you're not going to damage the airframe.  

When the overspeed condition is corrected (the speed has reduced to below Vfe by a set threshold) the flaps are returned to the selected position.