In blackjack, doubling down is when you double your bet and receive one more card after being dealt your first two cards. Doubling down is a strategy that can increase your winnings if used correctly. You can double down on your original two cards before the dealer gives you another card. There are three situations where it is advisable to double down:

1. When your cards total 11.
2. When you have a soft 16, 17, or 18 (this means that you have a card plus an ace) .
3. When you have a hard nine and the dealer’s face-up card is two-six (inclusive) .

It is important to note that you should never double down when the dealer has an ace. Additionally, you should avoid doubling down when your hand exceeds eleven. Doubling down is a risky move, and it is essential to get the balance right between playing it safe and taking the risk.