"Everywhere I looked people were collapsing. A lot of the Harpy supporters were using the Jelly-Brain jinx."
-- Timothy Blenckinsop, Puddlemere United Supporter (DP4)
Presumably affects the target’s mental processes.
References from the canon
During the September 1999 riot that took place during the Puddlemere/ Holyhead game, a lot of Harpy supporters were using this jinx (DP4).