The new perk is called Electric Cherry. In order to get it you will need to take the gondola up to the upper area of the prison. You have to power on the machine then buy it for 2000 points. When you reload it sends a shock out. If you use up all the ammo in the mag without reloading (force reload) the shock wave will be much bigger and has a higher chance to kill zombies. You will want to get a large group together and walk close to them and reload or fire off the last shot. If you have juggernog you have a higher chance of surviving the encounter since you have to be extremely close to them.
This is what the machine looks like.