The Imperius Curse presumably causes a magically induced partial paralysis of the neocortex and the prefrontal cortex, the parts of the brain which, respectively, control conscious thought/logic and decision making. This, in turn, would reduce the victim's capacity for independent thought and make them more receptive to outside influence. This would explain why the strength of mind is so vital for resisting the curse since the act of forcing oneself to realize one's actions are, in fact, not of their own doing when their brain says otherwise would be no mean feat.