The spell Expelliarmus is a defensive spell that attempts to disarms your opponent, causing their weapon (usually a wand) to fly out of their hand. It has been known to knock an opponent backwards in some cases. As seen in the movie version of Harry Potter and the Goblet of Fire, it can also knock back an opponent without disarming them.
It was first learned by Harry Potter by seeing Professor Snape using it in the Dueling Club that Lockhart started.
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