You know, that thing.
It's called simulating the frame. Some directors use viewfinders, but most use their hands. And as pretentious as it might appear, it's a necessary evil...because at the end of the directing day - it's all about what's in the frame.
The Frame: the perimeter of a TV/film picture; a single photographic unit of film.
We're talking composition here. First there's the dramatic composition of your story, made up by the plot and basic structural elements of set up, complication, rising action, climax, and resolution. And next comes the visual composition of that story.
That's where the director and his hands come in.
Here's a great article by R. Berden entitled Composition and the Elements of Visual Design. It's geared toward photography, but a lot of the rules and principles can be applied to composing images for film and television.
Because frames become shots that become scenes that become acts that become films or TV shows. But there are no actual pictures on the screen, only moving (pictures)...it all happens in the mind of the viewer.
Sometimes when you walk on a set, you'll get choked because it's not how you imagined. And there's light stands and ladders and dollies and cranes and dozens of crew people milling around...and you freak because you think there's no way you'll be able to compose the shots you want in order to convey the drama of the story. It won't look right.
Not to worry.
Hold up your hands and find the frame. And then put only what you want to be seen inside that frame. Everything around it won't matter. One of the most important things a director has to realize is that the viewer will fill in the blanks and 'create' in their mind everything else outside that frame, based on what they see within the frame. They'll 'see' what they're supposed to see. What they won't see is the real world.
So don't be afraid to hold up your hands. It's a director's way of finding the frame.
And when on set, it all starts with the frame.