Tuesday. Hot. That lot from across the marsh have been at it again. Playing a stupid game on their broomsticks. A big leather ball landed in my cabbages. I hexed the man who came for it. I'd like to see him fly with his knees on back to front, the great hairy hog.
-- Gertie Keddle's 11th Century diary (QA3)
This hex causes the target’s knee to become backwards. It seems to be a popular attack used when groups of wizards get into a fight, particularly when it comes to sporting competitions (Pm, BoS).
History and Notes
This hex has been in use for at least a thousand years, as evidenced by its mention in the 11th century diary of Gertie Keddle who lived on the edge of Queerditch Marsh (QA3).
The name given for this hex is not stated in canon.
A similar hex found is mentioned in the Daily Prophet newsletters:
"We’ve had broom accidents, we’ve had explosions, we’ve had charms that have backfired horribly," said Ministry spokesperson Stamford Jorkins this morning. "Wards at St. Mungo’s Hospital for Magical Maladies and injuries are never busier than at Hallowe’en. There’s still a wizard in there with his head on back to front from last year. We’re not trying to stop people having fun. We’re just trying to stop them turning each other into artichokes without checking to see if they know the counter-charm." (DP4).
From the Web
History of the Quidditch World Cup from Pottermore