Item Abilities - Passive Attacks

Item Abilities - Passive Attacks

I am creating a map which requires a type of item that attacks when in a unit's inventory, without the unit having to do anything. The best example of this is Battle Ships. Does anyone know how this was done?

This question is practically deserving of being in some kind of FAQ.

Give the item a Phoenix Fire-based ability, or make a modelless invulnerable dummy attack unit and move it to the position of the item-possessing unit every small interval of time.