This one was so amazing, it made me register just so I could say so.
Excellent, excellent, excellent!
I did slightly disagree with Alasdair's gloss on the meaning of the title, though. First and foremost, it's the "turn away" or "banish" meaning of apotropaic that seems most relevant to the story. The secondary "protective charm" meanings seem just that: secondary, and not hugely relevant to the story. The emphasis, the real horror of the story, lies in what is just about to happen to Janet.