"The usual," said Ron indifferently, demonstrating a rude hand gesture.
It was not like Malfoy to pass up the chance to demonstrate his power as prefect, which he had happily abused all the previous year.
Luna was demonstrating her usual knack of speaking uncomfortable truths; he had never met anyone quite like her.
Perhaps he had gained the idea from Professor Slughorn, the teacher with whom he was on best terms, who had demonstrated how influential a role a teacher can play.
Sure enough, McLaggen, for reasons best known to himself, had pulled Peakes’s bat from him and appeared to be demonstrating how to hit a Bludger toward an oncoming Cadwallader.
Harry was sure that Dumbledore was going to refuse, that he would tell Riddle there would be plenty of time for practical demonstrations at Hogwarts, that they were currently in a building full of Muggles and must therefore be cautious.
… I hope to be able to demonstrate them to you in due course.
There are no more uses of "demonstrate" in the book.