What does the "Project->Android Tools->Fix Project Properties" Eclipse command exaclty do?

I have read many times to use it, mostly in answers related with Android R generation problems in Eclipse, but for me it is like magic, I press on it and I suppose it Fixs some Project Properties, but nothing happens... no visual feedback, no info on which project properties has been fixed... anyone can give a bit of insight on what does it exactly do? 

Thx