An EMP or Electromagnetic Pulse grenade is a tactical grenade obtainable from the Mystery Box in Green Run. As the name indicates, the grenade emmits an electromagnetic pulse, having effect on both the living and the dead.
Zombies hit by the blast will be pacified, staying still or moving slowly and aimlessly until a player goes too close to them, sprints or shoots. This can be used to buy time for other activities without being in danger of the zombies. Zombies that are affected by an EMP do not have a blue glow in their eyes. This is caused by the fact zombies are revitalized by 115 due to it's highly electromagnetic properties. In addition, an EMP can interrupt signal, in this case being the Moon Pyramid Device's signal to the zombies. The EMP creates an explosion of electromagnetic noise that blocks radio waves and fries circuitry. So as long as the pulse is active, the controller in the MPD cannot communicate, as the frequencies are all blocked by radio noise, and the zombies lose their primary motivation to continue pursuing anything in particular.
Should a player be hit by an EMP, their HUD is disabled for the duration of the EMP, and any optical gunsights are nullified. If used while the Pack-a-Punch Machine is in use, it will abruptly stop upgrading a weapon. Perk-a-Cola machines (except Quick Revive when playing solo) are also disabled momentarily, and their respective perks are not in effect for any players carrying them until the machine is powered again. When an EMP grenade is used when the Mystery Box is in use, the box won't give the player a weapon and the player has lost it's money. Once again, the EMP interrupts the signal from the MPD, locking the controller out of his Aethereal access to that area, ergo, he can no longer 'insert' weapons or objects into that time via the portal known as the mystery box due to that disconnect. Note that the box is still there, the same way the guns you have previously purchased are still there because they have already been completely manifested and are now physical and out of the reach of the controller. However, the guns remain guns and the box remains nothing but a box. This effect effect on humans also makes it useful on Grief in order to weaken the enemy, and potentially drain them of points spent on the Mystery Box or Pack-a-Punch Machine as well as remove them from their weapons should the timing be right.