The EMP is a novus superweapon. When activated it triggers an EMP blast which shuts down any enemy units and structures including Hierarchy walkers. It doesn't affect infantry and deals no damage. Units affected by this are rendered completely harmless, since they can't move or attack. Not even walker are safe from this since it will affect them, as they are machinery. This way, the whole unit will stop working. if used on a super weapon, it will stop the countdown (I guess).