Always drink with your non-dominant hand — if you get caught using the wrong one, you must chug your drink.
Buffalo is more of a lifestyle than a game; it's a test of focus. Once you agree to the rules, you must always hold your drink in your non-dominant hand. If another player catches you drinking with the 'wrong' hand, they shout 'Buffalo!' — and you must finish your drink on the spot. This game can last for one evening, but for true aficionados, it never ends.
All players must agree to start the game. From that second on, the rules apply. The game lasts all evening, or potentially for a lifetime, depending on the agreement.
Once you confirm the game, you're in. There is no backing out of an active Buffalo contract.
Right-handers must drink exclusively with their left hand, left-handers with their right (always the non-dominant hand).
The only exception is holding the drink with both hands at once; in this case, you are safe and Buffalo does not apply to you.
If someone catches you with a drink in your dominant hand, they shout 'Buffalo!'. You must immediately finish the entire content of your glass or can.
If you shout 'Buffalo!' at someone who was drinking with the correct hand (or holding the drink with both), you must finish your own drink as a penalty.
The rules apply forever. If you meet a player ten years later at a wedding and they hold a drink in their right hand, you can shout Buffalo.