The option of Doubling down is considered as one of the most profitable options. Player may choose to Doubling down in case he strongly believes the extra card (ONLY ONE) will make his hand the winning one. When Doubling down player doubles his initial bet and receives one extra card.
By increasing the bet in an advantageous situation, player may gain much more profit as compared to the usual. If you want to Double Down in a face-down game, you should toss the cards face up on the table and add the additional bet next to your initial one. Then, the dealer deals the extra card. In a face-down game, the dealer deals another card without showing it. He simply tucks the card face down under your bet to be revealed a bit later.
As a rule, players may double down for any amounts up to the size of the initial bet. When you decide to Double down for the amount of money that is less than the original bet, this is called Doubling Down for less. However, you must be absolutely positive when choosing to Double Down, as you will be dealt ONLY ONE more card and your hand stands automatically.