It happens when one side/the affected limb, etc., can't take the bruises, bumps (the beating), anymore, and instead of pain going away, it keeps repeating itself. The result of numbness, tingling pain, stiffness, tremors (shaking of the limb), color changes in the skin, temerature changes in the affected limb, and more.
It can also cause insomnia, migranes, nausia, etc.
To cut it short:
A pain in the ass.
some guy 2:Yeah?
some guy 1:My RSD is flaring up, I have to go home
some guy 2:That stinks.
some guy 1:Like you'd know! Bastard.