Use the "Event" button on the indicated stage to get the corresponding result.

Cemetery: Divine ray kills zombies.
Drive-In: Plays music that makes zombies dance briefly.
Military Base: Line of TNT drops down.
Secret Lab: Three lasers rotates around the middle of the level that will kill everything.
Suburbia: UFO abducts zombies.
Woods: Rain slows down zombies; torch is extinguished and cannot set zombies on fire, but can still scare them; zombies that are already on fire remain burning.