21 Burn Blackjack is a blackjack variant where the player can burn their second card. But, the player has to make a negative side bet, equal to half his original wager, on the replacement card to do so. You can get a house edge of 0.54%.
The player can expect to lose 67.31% of whatever they bet on, before considering any benefit to the blackjack wager. You should only invoke the Burn option when it stands to improve the value of the blackjack wager by 67.31% * 50% = 33.65%.