In The Turn of the Screw, the title itself makes use of an idiom that, in most cases, is used metaphorically to refer to something excruciating or horrible. In this case, it's used to describe a story about ghosts haunting children. It's also used as a way of expressing a certain escalation of the stakes, as Douglas listens to a story about the haunting of a child and then says he has an even worse story:
"But it's not the first occurrence of its charming kind that I know to have involved a child. If the child gives the effect another turn of the screw, what do you say to two children—?"
The idea here is that a story about ghosts haunting two children (instead of one) is almost inexpressibly horrible, adding yet another "turn of the screw."
Furthermore, at the end of the novella, the governess uses the idiom when pontificating about her duty to face the “unnatural” situation at Bly. She sees it as her responsibility to protect the children from the ghosts, and to make herself feel better about the circumstances, she reasons that, if this whole ordeal with the ghosts is “unnatural,” it’s “only another turn of the screw of ordinary human virtue.” In other words, she tells herself that facing things that are wicked and “unnatural” merely requires people to embrace the ugliest parts of human nature itself. Dealing with such depraved situations is, according to the governess, just one more "turn of the screw."
There's also some pretty gruesome connotations to the idiom, since "turn of the screw" is sometimes understood to have come from the language of medieval torture. Many of these old torture methods relied on the torturer cranking or screwing something up a notch and, in doing so, causing various torture implements to tighten or more intensely pierce the victim. There is, then, a ruthlessness to the expression, and this infuses the narrative with a sense of extreme discomfort.
The governess opens her tale with a rather strange but nonetheless evocative metaphor comparing the beginning of her time at Bly to a strange kind of oscillation:
I remember the whole beginning as a succession of flights and drops, a little see-saw of the right throbs and the wrong.
Given the ambiguity of the entire story (especially regarding whether the ghosts are really there), this is an appropriately vague but visceral metaphor. The phrase "a succession of flights and drops" is very evocative, bringing to mind the constantly changing motion of a "see-saw" and, in turn, beginning the governess's tale with a sense of instability. But the phrase "the right throbs and the wrong" is a little less tangible, as it's unclear what these "throbs" are or what, exactly, is supposed to be "throb[bing]." Still, the idea of some sort of pulsation that can be either positive or negative does seem to evoke the feeling of life suddenly going right and then unexpectedly going wrong. What's more, "throb" has obvious bodily connotations, as if an injury is pulsing with pain, and this sense of an unpleasant emanation seems relevant to the uncanny, unidentifiable terror the governess feels while living at Bly.
Soon after arriving at Bly, the governess takes stock of her situation, using both a metaphor and a simile to express her feelings about the somewhat ambiguous and daunting circumstances of her employment. First, she metaphorically compares her "circumstances" to something huge, imposing, and intimidating:
They had, as it were, an extent and mass for which I had not been prepared and in the presence of which I found myself, freshly, a little scared as well as a little proud.
By presenting her situation at Bly as such a colossal undertaking, the governess contributes to the ominous and slightly overwhelming quality of her initial days in this new place. And yet, nothing has really happened yet; the governess simply senses the enormity of the task ahead of her, but she has no idea just how intense this experience will truly be—once she starts seeing ghosts, her situation really does become quite daunting.
To that end, the governess uses a simile to further characterize what it feels like to live in the huge mansion at Bly:
[...] I had the fancy of our being almost as lost as a handful of passengers in a great drifting ship. Well, I was, strangely, at the helm!
In this simile, the governess compares her situation at Bly to the experience of being on a large, wayward ship. This is an especially insightful simile because the governess is not only on a ship that seems to be lost—she is lost aboard the ship itself. And yet, she's the one supposedly steering it. This speaks to the deep sense of confusion she experiences when she first arrives at Bly, where she's expected to be more or less in charge and yet doesn't even understand why previous governesses have failed in her position, nor why her employer refuses to correspond with her.
In a conversation with Mrs. Grose about how to handle children, the governess metaphorically presents bad behavior as some sort of contaminant, as if it's infectious and has the power to corrupt others:
"You like them with the spirit to be naughty?" Then, keeping pace with her answer, "So do I!" I eagerly brought out. "But not to the degree to contaminate—"
"To contaminate?"—my big word left her at a loss. I explained it. "To corrupt."
She stared, taking my meaning in; but it produced in her an odd laugh. "Are you afraid he'll corrupt you?"
The metaphorical idea here is that bad behavior is contagious and has a very tangible impact on other people. This highlights the governess's fear that she might not be up to the task of caring for Miles and Flora. It also hints at her growing—but as of yet unarticulated—fear that there's something wrong with the children and that there's a reason nobody else has agreed to educate them since their last governess died.
By the end of the book, though, it's not all that clear whether the children "contaminate" the governess or if it's perhaps the other way around. Though the governess is terrorized by the idea that the children are in cahoots with the ghosts, it's possible that she has deluded herself and, in doing so, has ultimately "contaminated" the children with her fear. In this reading, the metaphorical idea of contamination is especially relevant, as Miles ends up dying under ambiguous circumstances—in keeping with the metaphor of contamination, it's arguable that he has been fatally infected by the governess's fear. If this is the case, then it's a good example of situational irony: the governess, who originally worried Miles might "contaminat[e]" her with his bad behavior, ends up "contaminat[ing]" him with her fears and anxieties.
As she tells her story, the governess considers the period just before everything started to go wrong at Bly—that is, the period right before she started seeing the ghosts of Peter Quint and Miss Jessel. She sees this stretch of time as rather idyllic, but she also retrospectively sees it as the calm before the storm, and she uses a metaphor to highlight this feeling:
It may be, of course, above all, that what suddenly broke into this gives the previous time a charm of stillness—that hush in which something gathers or crouches.
The governess metaphorically compares the peacefulness of her initial days at Bly to an ominous kind of "stillness," suggesting that the quiet, idyllic circumstances have taken on a sense of danger in her memory. The impending terror she experiences is presented as a terrible animal about to spring upon her—a metaphor that emphasizes just how overpowering the ghosts are when they do appear, as well as how unprepared the governess is to encounter them.
To really drive this point home, the governess goes on to add a simile, saying, "The change was actually like the spring of a beast." This comparison emphasizes a sense of powerlessness, as if the governess couldn't have possibly done anything to protect herself from the antagonism of the ghosts.
Upon seeing the ghost of Peter Quint for the second time, the governess uses a metaphor to describe the experience of staring straight into the ghost's face and somehow knowing that he's looking for one of the children:
The flash of this knowledge—for it was knowledge in the midst of dread—produced in me the most extraordinary effect, started, as I stood there, a sudden vibration of duty and courage.
The governess's strange understanding (if it can be called that, since it's unclear at the end of the novel whether or not she's a reliable narrator) of the ghost's intentions is metaphorically portrayed as a "flash," bringing to mind a sudden, overpowering, blinding sense of certainty. In some ways, this sets readers up to unquestioningly take the governess at her word, as she implies that her interpretation of the situation rests on an instant and all-encompassing understanding of what's going on—and yet, it's actually quite farfetched to claim an immediate understanding of any situation, let alone a supernatural situation that involves assessing the intentions of an unspeaking ghost.
This passage also metaphorically presents the "knowledge" of Peter Quint's true intentions as "a sudden vibration of duty and courage." In other words, the governess is affected to her very core by seeing Quint and determining his intentions, and the fact that this leads to a sense of "duty and courage" underscores the idea that a protective impulse overtakes her—it's as if she has a visceral urge to shield the children from horror.
After the governess sees the ghost of Miss Jessel, she breaks down. Later, though, she returns to Flora, and she uses a metaphor in her narration to describe the calming effect Flora has on her:
I had simply, in other words, plunged afresh into Flora's special society and there become aware—it was almost a luxury!—that she could put her little conscious hand straight upon the spot that ached.
There are actually two metaphors at play in this passage. First, there's the idea that the narrator can "plunge[]" into Flora's "special society." This presents Flora herself as the kind of person who can completely envelop people in her company, and though the governess seems to delight in the feeling of "plung[ing] afresh" into Flora's influence, this idea does have some rather sinister implications, suggesting that Flora is a vortex of sorts. This little girl, in other words, is the sort of charming person who puts others so much at ease that they forget to worry about anything.
Second, the governess metaphorically portrays her own worry about the ghosts as a physical ailment—a "spot that ached." Again, Flora apparently has a soothing presence, putting "her little conscious hand straight upon" the governess's invisible wound and, it seems, healing her right away. But this isn't necessarily a good thing; readers don't yet know whether or not Flora can be trusted, so the fact that she has such sway over the governess is a little disconcerting.
To that end, when the governess tells Mrs. Grose about being out with Flora and seeing Miss Jessel's ghost, she insists that Flora's beautiful charm in the aftermath of the supernatural event was actually quite calculated. The governess tells Mrs. Grose in this conversation that Flora displays a "perceptible increase of movement" and a "greater intensity of play" simply to hide the fact that she has seen the ghost. And yet, the governess also thinks that Flora's behavior is aimed at trying to detect whether the governess herself sees Miss Jessel, too. In this way, the governess appears a bit more wary of Flora than her initial metaphors about the little girl might indicate. She is, it seems, aware of Flora's strong influence over her, and she is unsure of whether or not she should fully trust her, ultimately suspecting that Flora is hiding something from her. All in all, the metaphors the governess uses to describe Flora in this section underscore her suspicion that the little girl's charming qualities have potentially sinister implications.
