Starvation mode...myth or fact?

ok, so i'm sure there is scientific evidence to back up 'starvation mode', BUT i find it confusing when everything you read says, 'if you starve yourself, you won't lose weight. your body will hold on to every calorie.' if this is true, how do anorexics become so thin??? obviously, if you eat only a couple hundred calories, as many suffering from anorexia do, you will lose weight.... it's undeniable. so, what's the scoop? btw.... i'm not aspiring to starve myself. i'm asking from a scientific & medical standpoint.