"If the child gives the effect another turn of the screw"
A Turn of the Screw - Credit: Rasbak
This is the first of two references to the novella's title. Here, the phrase evokes a heightening of dramatic tension. In the second, where the governess reflects that the ordeal she has to go through requires "only another turn of the screw of ordinary human virtue", it implies a redoubling of personal effort. This shift in meaning illustrates in microcosm the blurring of art and reality that infuses the whole text.
It also provides a key to understanding the governess's role in what ensues: if turning the screw is a metaphor for an author's manipulation of the reader, then the governess's own later use of the expression is an indication that we as readers should be alert to the creative contrivances she uses to delude both herself and us.