What words trigger the screen effects in iMessage?
Happy Birthday: Balloons
Congratulations: Confetti
Happy New Years: Fireworks
Happy Chinese New Years: Celebration
Pew pew: Light/Laser Show

Also if you hold down the send button you can access a set of effects you can add to your message like a (Slam/Loud/Gentle/Invisible Ink). Now if you hold down the send button, not only do you get those set of effects, but the effects above with the SHORT list of trigger words are accessible at the top of the screen.