This hex is poorly described. We know it causes snot in your nose to turn into bats (of unknown size), and to possiblyattack you in some way. Per secondary cannon, this is a true transfiguration, that is the snot becomes black bats. It remains unclear if the bats fly away (as a true bat would) or attack. Given the fear that the Weasley brothers have for this spell, I think they must do more than just fly away.
Found in
Primary Cannon. : Mrs. J. K. Rowling. Harry Potter and the Goblet of Fire Kindle Location 4204. © 2003 Pottermore Limited. American Kindle Edition.
Secondary Cannon. : magicscrapbook. “‘Book of Spells’ transcript” Last Viewed 2022-07-25