"The core concept of this lesson is this-your Shield Charm shouldn't be a wall. It should be a net."
Flitwick drew two diagrams on the blackboard.
On the left was a flat panel labeled "fixed-frequency shield."
On the right was an interwoven grid labeled "adaptive-frequency shield."
"A fixed-frequency shield can only block attacks with frequencies close to its own. An adaptive-frequency shield can automatically adjust its vibration pattern according to the frequency of the incoming spell."
He put down the chalk.
"This is why you need to understand frequency. Not to pass an exam. To stay alive."
The classroom door opened just then.
Douglas walked in.
He wore that old sweater, its sleeves rolled up to his forearms, carrying an insulated cup in his left hand and a stack of parchment covered in writing in his right.
He walked to the left side of the blackboard and stood beside the anatomical diagram of the brain.
"Professor Flitwick has finished explaining frequency."
Douglas glanced around the classroom.
"Now I'll explain what happens to your brain when the frequencies don't match."
He set his insulated cup on the edge of the teacher's desk, pulled a sheet from the stack of parchment, and tapped it with his wand.
The writing on the sheet enlarged and floated in the air.
It was an excerpt from the abstract of a paper by Bellatrix Lestrange.
Of course, the author's name had been blacked out.
"This passage comes from an anonymous senior researcher in dark magic."
Douglas spoke flatly, as though reading out a shopping list.
"She-or he-described in detail the entire pathway by which the Cruciatus Curse attacks the human body's pain fibers."
He drew a line in the air with his wand, extending it from the first paragraph of the floating text to the anatomical