Instructions
Look around the "Nazi Zombies" maps and keep an eye out for the Insta-Kill icons that resemble skulls. Extra icons are dropped by the zombies that you kill.
Step up to one of the icons to "pick" them up and activate them. Do not walk up to one unless you want to activate it.
Kill as many nearby zombies as you can before the 30 seconds are up. When the time is up, you will hear a "vanishing" sound.