First, the extension installer gives you an option in your New Wave menu to “Go Trick or Treating”. When you click that, it creates a new wave and inserts a gadget (try clicking around that to see what surprises await). Then, whenever a user types ‘trick or treat’, the robot fetches an image from Google Image Search for either a yummy candy bar, or well, something not that yummy.
From the Google Wave Developer Blog\
No one I know has ever celebrated Halloween (it’s relatively new in Australia), and I say “Bah humbug!” (wrong holiday I know). But for those of you who want to get into the spirit of it (bad pun I know), but don’t want to leave Wave, this might be for you.