What does the ex pression "turn of the screw ...

Hello annie, "Turn of the screw" is a novel written by Henry James. I'm not sure what does the expression mean in contemporary slang, but you may look

I believe the "turn of the screw" in fact refers to an old torture apparatus, where each turn of the screw amplified the pain of the subject.  Here are a couple of examples from literature where this meaning seems clear:

1. "Just when it seemed to him that another turn of the screw would kill him, little Stanislovas stopped."   from The Jungle by Upton Sinclair

2. "He perceived himself so extremely forlorn and lamentable, and was moved so deeply by the oppressive sorrow, that another turn of the screw, he felt, would bring tears out of his eyes."   from  Tales of Unrest by Joseph Conrad

 I hope that helps, Annie.

"What's your title?" - 'The Turn of the Screw.' Studies in Short Fiction - Find Articles
""Turn of the screw" is of course a metaphorical expression that signifies an unpleasant or disconcerting turn of events. However, the idiom derives from the torture chamber, where its meaning was all too literal. The "screws" were the grim instruments of torture that jailers (who also came to be known as "screws") applied to the limbs of their captives in order, usually, to extract the appropriate confession."

http://findarticles.com/p/articles/mi_m2455/is_n1_v30/ai_14085491/pg_6

in james context, it is an image for the pressure of social standards. a common theme for james.