So you just decided to try out an unknown, handwritten incantation and see what would happen?
Yes, those who progress in using magic without shouting incantations gain an element of surprise in their spell-casting.
A reasonable amount of cheating ensued; many people were merely whispering the incantation instead of saying it aloud.
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.
One simple incantation and you will enter a top-quality, highly realistic, thirty-minute daydream, easy to fit into the average school lesson and virtually undetectable (side effects include vacant expression and minor drooling).
Harry frequently looked over at his classmates in the common room or at mealtimes to see them purple in the face and straining as though they had overdosed on U-No-Poo; but he knew that they were really struggling to make spells work without saying incantations aloud.
Harry reacted instinctively; his wand was out of his pocket and the incantation sprang to mind without conscious thought: Le\icorpus!
Pushing Harry roughly aside, he knelt over Malfoy, drew his wand, and traced it over the deep wounds Harry’s curse had made, muttering an incantation that sounded almost like song.
He had just found an incantation "Sectum-sempra!" scrawled in a margin above the intriguing words "For enemies," and was itching to try it out, but thought it best not to in front of Hermione.
Harry had not yet managed to bring off the Re-filling Charm without saying the incantation aloud, but the idea that he might not be able to do it tonight was laughable: Indeed, Harry grinned to himself as, unnoticed by either Hagrid or Slug-liorn (now swapping tales of the illegal trade in dragon eggs) he pointed his wand under the table at the emptying bottles and they immediately began to refill.
There are no more uses of "incantation" in the book.
Show samples from other sources
She believes in the power of magical incantations.
Harry Potter says the incantation, "expecto patronum" as he focuses on a happy memory to summon protection from dark magic.