The Disarming Charm (incantation: Expelliarmus) is a defensive charm that can force the victim to release whatever they were holding at the time, usually against his or her will. It is common to see this spell used in duels, to make an opponent release their wand.
The Disarming Charm always appears as a jet of red light; indeed, this is one of the things that makes it recognisable before the effects of the charm are seen.
The Disarming Charm causes whatever an opponent is holding at the time - usually a wand - to fly high out of their reach; however, more enthusiastic casts can result in the victim of the charm being knocked unconscious in a similar manner to the Stunning Spell. If done too weakly, however, it will merely blow them back a few paces, or make their hair stand on end. If one mispronounces the spell's incantation as Expellimellius, the victim's sleeves will be set aflame.
If two wands with the same core are forced to do battle with each other, the effect is a magical connection called Priori Incantatem.