The Shrink Ray was a weapon that shrunk things like cars, crates, boxes, containers, and Humans. You couldn't shrink buildings, bosses, Blasto, yourself, or the game floor. Also, you couldn't PK anything that got shrunk. Once a human or enemy shrunk, they did not attack you. Humans ran away in terror and Tanks just stopped what they were doing. It was only obtainable after you completed Destroy All Humans! Big Willy Unleashed and selected it from the Cheat Menu.