All players reveal a card from their hand simultaneously and discard it unless all other revealed cards have a lower cost.
Then each player draws a card; if you did not discard, you may instead gain a Prize (from the Prize pile) or a Duchy.
When playing with this card in a Kingdom set, add the Prizes from the Cornucopia expansion.
All players, including you, reveal a card simultaneously from their hands. Everyone chooses one of their cards in secret and then, when all are ready, players reveal their chosen card at the same time, and the cost of the revealed cards is compared.
If the cost of their card is strictly higher than all other revealed cards, that player is said to have 'won' the prize fight and keeps their card, returning it to their hand - everyone else must discard their revealed card. It is possible to have no winner, if there is a tie and none of the cards has a higher cost than all the others, in which case all players discard their revealed cards. When playing with cards from Alchemy, mixed costs of Coins and Potions must be considered. These card costs might not be strictly comparable, so here too, there could be no winner.
For example, if one player reveals a Silver and another player reveals a Transmute, there would be no winner.
Next, all players draw a card. So most players would end up with the same size hand they started with, but a player who 'won' the Prize Fight would end up with a net +1 Card.
If you were the winner, you may opt to not draw a card at this point, and instead gain a Prize or Duchy card. Unlike Tournament, the gained Prize goes into your discard pile, rather than on top of your deck.