Rules were:
Register your deck so you use the same deck throughout the tourney.
Change your avatar to your home flag while playing a tournament match.
Don't spy on "flagged" tournament matches (your opponent's decks).
The registering and "don't spy" were so that you couldn't custom-pick a deck to destroy your opponent, or know their strategy in advance. But I suppose it can work either way -- if no one registers, then you don't know in advance which deck you are going to face.