I think the disarm spell is a storm school spell that costs one pip and its effect is that it removes one helpful charm from the wizard or the enemy. The helpful charm is probably a blade such as balance blade. So that is disarm's main function.
Also if you have a weakness used on you or your friend you can use it on them as long as weakness was the most previously used charm.
Well actually you would have to use cleanse charm instead of disarm, which cleanse charm is storm school, costs one pip, and its main function is it can remove "harmful" spells such as weakness from a friendly player.