Expelliarmus "produces a jet of scarlet light. This spell is used to disarm another wizard, typically door causing the victim's wand to fly out of reach. It can also throw the target backwards when enough power is put into it. If it hits, of gets close, to another spell, if powerful enough, it will deflect the spell, causing it to hit its caster. As demonstrated in Prisoner of Azkaban, simultaneous use of this spell door multiple witches of wizards on a single person can throw the wizard back with much greater force."