He set off again around the other side of the classroom toward his desk, and again, they watched him as he walked, his dark robes billowing behind him.
“…you are, I believe, complete novices in the use of non-verbal spells. What is the advantage of a non-verbal spell?”
Hermione’s hand shot into the air. Snape took his time looking around at everybody else, making sure he had no choice, before saying curtly, “Very well—Miss Granger?”
“Your adversary has no warning about what kind of magic you’re about to perform,” said Hermione, “which gives you a split-second advantage.”
“An answer copied almost word for word from The Standard Book of Spells, Grade Six,” said Snape dismissively (over in the corner, Malfoy sniggered), “but correct in essentials. Yes, those who progress in using magic without shouting incantations gain an element of surprise in their spell-casting. Not all wizards can do this, of course; it is a question of concentration and mind power which some”—his gaze lingered maliciously upon Harry once more—“lack.”
Harry knew Snape was thinking of their disastrous Occlumency lessons of the previous year. He refused to drop his gaze, but glowered at Snape until Snape looked away.
“You will now divide,” Snape went on, “into pairs. One partner will attempt to jinx the other without speaking. The other will attempt to repel the jinx in equal silence. Carry on.”
Although Snape did not know it, Harry had taught at least half the class (everyone who had been a member of the D.A.) how to perform a Shield Charm the previous year. None of them had ever cast the charm without speaking, however. A reasonable amount of cheating ensued; many people were merely whispering the incantation instead of saying it aloud. Typically, ten minutes into the lesson Hermione managed to repel Neville’s muttered Jelly-Legs Jinx without uttering a single word, a feat that would surely have earned her twenty points for Gryffindor from any reasonable teacher, thought Harry bitterly, but which Snape ignored. He swept between them as they practiced, looking just as much like an overgrown bat as ever, lingering to watch Harry and Ron struggling with the task.
Ron, who was supposed to be jinxing Harry, was purple in the face, his lips tightly compressed to save himself from the temptation of muttering the incantation. Harry had his wand raised, waiting on tenterhooks to repel a jinx that seemed unlikely ever to come.
“Pathetic, Weasley,” said Snape, after a while. “Here—let me show you—”
He turned his wand on Harry so fast that Harry reacted instinctively; all thought of non-verbal spells forgotten, he yelled, “Protego!”