I can't remember who taught this to me, but I've brought it to whatever gatherings I could, and after a quick internet search failed to reveal a write-up of the rules, I decided I'd take a shot at it. Perhaps it's called something else by other people, I'd be happy to learn that. I should also add some illustrations, but that's not my forte.
WHIZ, BANG, BOOM.
Sit in a circle (preferably around a campfire). It should be obvious who is to your left and to your right. Each person should also be able to point to someone across the circle from them.
OBJECT OF GAME.
To socialize and have a good time. The game ends when everyone is bored, or unable to continue playing.
HOW TO PLAY.
An easy way to think of the game is to imagine you are passing an imaginary hot potato from person to person around the circle. Anytime anyone screws up, they take a drink.
A "starter" is selected using any method.
The starter begins a round by performing a Whiz to either their right or left. This initiates the movement of the imaginary potato by sending it in a direction around the circle.
The imaginary potato is passed from player to player. When receiving the potato a player must choose and perform one of three possible moves - Whiz, Bang, or Boom to continue passing the potato. If the receiving player makes a mistake, either by performing an illegal move, performing the move incorrectly (typically by using the wrong hand), or taking too long as judged by the group, then the round ends. The offending player takes a drink, and a new round beings with the drinker as the new starter.
The Whiz is used to either a) start a round, b) continue passing the potato in the same direction as it was received, or c) establish a new direction for the potato if it came from across the circle as a result of a "Boom". A Whiz cannot be used to change direction.
To perform a Whiz, a player makes a sweeping motion with an arm and hand from outside to inside, while saying "Whiz" out loud.
That is, if the potato comes to the player from their right side, they sweep their right arm and hand from right to left, resulting in them pointing left.
This continues passing the potato leftward, to the person at the players left.
If the potato comes at a player from the left side, they must sweep their left hand and arm from left to right, while pointing (and saying "Whiz"). This would continue the potato's motion rightward, passing it to the person at the players right.
A Whiz cannot be used to change direction. If the potato arrives from the left, the player cannot use their right hand to sweep right-to-left and redirect it leftward. Changing direction constitutes a drinkable, round-ending mistake.
A common mistake is to sweep in the correct direction, but using the wrong arm. If the potato arrives from the right, a player cannot use their left arm and attempt to sweep right to left. The right arm must be used.
The potato may arrive at a player from across the circle due to a Boom, in this case either a right or left sweep is permitted to direct the potato around the circle (the correct outside-to-inside sweeping motion must be performed, i.e. the left hand cannot be used to sweep right-to-left)
A Bang may only be performed in response to a Whiz, or another Bang. The bang is like a mirror that reflects the potato back to the person from whom the player received it.
The Bang is performed by holding up the opposite hand from which the potato arrived (either the right or left hand), palm facing the direction you are deflecting the potato back to, while saying "Bang."
Example: if the potato is arriving from a player's right, they would hold up their left hand, palm facing right, and say "Bang." This changes the direction of the potato, sending it back to the right of the player (where it came from).
If the player attempts to Bang with the wrong hand, that constitutes a drinkable, round-ending mistake.
A Bang often catches people off-guard who expect the potato to continue in the same direction after a few comfortable Whizzes. Two players can also get into a tense "Bang-off" by repeatedly Bang-ing back and forth until one of them Whizzes it along, or Booms it across the circle.
A Boom may only be performed following a Bang. The Boom sends the potato across the circle to any non-adjacent player (typically unexpectedly).
To perform a Boom, the player receiving the potato as a result of a Bang clearly points across the circle to anyone that is not immediately adjacent to them while saying "Boom".
Either hand may be used to point. If the receiving player is not clearly pointed to, that constitutes a round-ending, drinkable mistake.
A TYPICAL ROUND
1) The starter uses their right hand, sweeping right to left, and points left saying "Whiz".
2) The person to their left keeps the momentum going by saying "Whiz" and also sweeping their pointing right hand from right to left.
3) the person to their left holds up their left hand, palm facing right, and says "Bang".
4) the person to their right (who was the Whizzer in step 2) sweeps their left hand from left to right, pointing right, and says "Whiz"
5) the person to their right (who was the starter) holds their right hand up, palm facing left, and says "Bang" to reflect it back
6) the person to their left (the same poor Whizzer as in step 2, now feeling caught in the middle) says "Boom" and points across the circle at someone.
7) that person isn't paying enough attention, and they fail to Whiz properly. The round ends, that person drinks, and they start the next round with a Whiz in either direction.