What exactly is the criteria for the "Best Director" award?

This is something I've wondered about for some years, but I've never known who to ask. So after having watched the Oscars last night, I thought I'd ask you guys!

So I can understand how other awards for film can be judged (i.e. cinematography, acting, production design, etc.), because those are elements of the film that you actually see when you watch the film. But how does somebody judge directing? It seems like directing is something that would have to be witnessed during a film's production (instead of after, as is possible with the other awards) in order to be judged. Does the award have to do with directing styles? Is it some sort of meta-award for the movie? I'm just curious as to what the criteria for the award are, and what it means when a director wins this award. Thanks!