Silent magic and incantation-free magic were different things.
The former simply involved using spells without making a sound. It did not reduce the time needed to cast them, and still required a wand. It was part of the basic curriculum for fourth- and fifth-year young wizards before they mastered Nonverbal Spellcasting.
The latter was covered in the sixth-year Advanced Class. It required only the assistance of a wand and could simplify the hand movements involved in wielding it to a certain extent. It was the sort of thing that, in theory, every student beyond sixth year should know, but in practice, few truly mastered.
At that moment, the nearly transparent silhouette behind Harry gently waved a wand. This was clearly silent magic. The magic settled upon Harry without a sound. The Imperius Curse produced no magical light or visible signs to begin with.
Harry's body paused slightly, and then he stood rooted to the spot.
The silhouette behind him slowly circled around to his front. Seeing Harry's somewhat vacant face, he seemed to nod in satisfaction.
The next moment, Harry began to walk slowly forward. His movements were slightly uncoordinated at first, but soon became no different from usual. The silhouette continued to follow silently behind him.
Only this time, Harry no longer glanced around, but headed straight for the third floor.
The third floor held a large concentration of classrooms. There were no classes at the moment, and with the Duelling Club in session, most students were in the Great Hall. Most of the classrooms were empty.
Harry entered a very out-of-the-way classroom and stood inside.
The silhouette behind him revealed his true form then as well. Naturally, it was Draco Malfoy.
"Heh, I thought Potter was supposed to be so impressive. Is that all he's got? Didn't my