It's the "tickle charm".... Harry used it against Draco Malfoy during the his fight against him in the dueling club, causing Malfoy to resort to "Serpensortia", revealing Harry's status as a parslemouth in the process. (In the movie Harry also uses Rictus Empra in the dueling club, but in stead of a tickle effect, it launches Malfoy through the room).
In the computer games based on Harry Potter, Lockheart will teach rictus empra to Harry during your class Defense Against the Dark Arts, and during the remainder of the game, that spell is then your weapon against most of the enemies you'll meet during the game. In the game of book #2, you can also join the dueling club in the game, where rictus empra is one of the three allowed spells (the other two being mimblewimble (a spell which muffles your speech) and expelliarmus (which in the game bounces an enemy spell back, rather than disarming)).
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