The extreme close-up is a shot that would only show an eye, mouth or portion of an actors’ face. This form of shot produces a sense of importance in the minute, the apparently insignificant and normally imperceivable. From a normal range we may not see any emotion on a face, however cutting to an extreme close-up a welling up of tears in the eye of the apparently emotional-less face could be shown. The extreme close-up allows a director to reveal something normally unseen; the extreme close-up imbues the image it shows with a sense of importance. In Blade Runner an extreme close-up of an eye in the introductory scenes leads one to wonder about the significance of perception and leads the audience to become reflexive about their own participation in watching film. In an action film a director may show an extreme close-up of a spring flicking to indicate the small mechanical processes which lead to the large subsequent explosion. The extreme close-up ensures, because it fills the scene with a minute aspect of a larger whole, a sense of importance and significance is evoked. The extreme close-up also becomes a revealing technique: a quiet whispered secret rather than a loudly proclaimed revelation.
Here’s a link to an article which explores Hollywood’s view of the close-up.
The Motley View 
A great example of the ECU, of which most of my entries are about, is Carl Th. Dreyer. His film the Passion of Joan of Arc is comprised almost entirely of ECU’s brilliant psychological and emotional effect!
Darren Aronofky loves a good ECU. Check out the shot of Marion’s pimps lips in Requiem for a Dream, just before the sex party starts. The effect is way too intimate, like pressing the audiences face up againast the screen. I used to love R4AD, but increasingly it feels like a hysterical rant, desperate in its urge to proapagandise and terrify. Not a million miles from Exploitation Cheese like ‘Reefer Madness’. Lovely Blog BTW