In many films and TV shows, just before a seminal moment (though not necessarily before a climax), the director will call for a character's face to be half in darkness, half in light.
I first noticed this technique in 'The Empire Strikes Back'. During the lightsaber scene between Luke and Darth Vader, there is a moment when Luke is pushed back, underneath a stairwell, and half of his face is in darkness, half is in light.
Last Sunday, I saw it again in 'Boardwalk Empire'.
Just before Eddy Kessler jumped off of the balcony, he looked in the mirror. Half of his face was in darkness, half was in light.
What is the name of this technique? And what is the director trying to get across?