In blackjack, "double down" is a betting option that allows a player to double their original bet in exchange for committing to receive only one more card. This option is typically used when the player believes they have a strong chance of improving their hand with just one additional card. It is most often employed when the player has a total of 9, 10, or 11, as these totals are considered favorable for getting a high-value card to complete the hand. However, the player is committed to the increased bet and cannot take any more cards after doubling down.