its tough to get used to. if you only aim in their general area you'll only pull them. if you aim closer to the enemy it'll 'disable' the weapon for a second. if you aim it well (I haven't found the correct spot yet, perhaps due to lag in multiplayer) it'll pull the weapon.
the easiest way to pull it without worring about aiming your pull, is to pull them so they touch you, usually when they get to that point the weapon is pulled.