Expeliarmus

This spell is used to disarm another wizard,typically by causing the victims wand to fly out of reach. It can also throw the target backwards if enough power is put into it. If it hits or gets close to another spell and 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 by multiple witches or wizards on a single target will throw the wizard back with much greater force.