You just have to 'check-in' at any Hungry Jacks store and shake your phone to win. Every shake is a winner. Prizes range from a free soft serve to the Whopper.
You will need a facebook account.
Read the link for more info.
To bad not on android, yet.
Apparently you get locked out for 10 hours after each use (thanks to leon for the info).
Edit: As per the comments, app only works on the Iphone.