"Wrackspurt got you?"
-- Luna Lovegood (HBP7)
Wrackspurts are invisible things which some people believe float in though a victim’s ears and makes his or her brain go fuzzy (HBP7).
Luna asks Harry if a wrackspurt has got him and she explains "I thought I felt one zooming around in here." (HBP7)
wrack Eng., chiefly Scottish The roots of weeds, especially loosened from the soil (NSOED)
Draco put Harry into a Full Body-Bind under his Invisibility Cloak on the Hogwarts Express, and he couldn't move or call out (HBP8). In the book, Nymphadora Tonks found and rescued him. But in the Half-Blood Prince movie, Luna opens the train door and uses her Spectrespecs to see Harry's Wrackspurts -- "your head is full of them" - and then uses "Finite" to end Draco's spell. She then uses Episkey to fix his bloody nose, a job also given to Tonks in the book (HBP/f).