• At the start of the game, everyone is dealt a card.
• You can see your own cards.
• You cannot see other players' cards until you either fold or the game ends.
• Choose between high or low.
• Among those who chose "high", the player with the highest card wins.
• Among those who chose "low", the player with the lowest card wins.

Even if another player has the highest/lowest card, they won't be included unless they actually declare high/low. Therefore, declaring high/low becomes the key to winning.
Of course, if you are the only one who declares high/low, you win by default.