"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.