A move in Blackjack
. From the Blackjack FAQ:
To double the initial bet and receive exactly one more card. The option to double is often allowed on the players first two cards only, although some casinos allow doubling after splitting a pair. Many Northern Nevada casinos allowing doubling only with a two-card total of 10 or 11. It is very rare to find games that allow doubling of hands that have more than two cards.
Double down is very favorable to the player, and is offered in just about every Blackjack game out there. In order to minimize the house advantage, make sure you learn when to double down.
Basically, as with most Blackjack strategies, you will assume you get a ten-valued card (since 5/13 of the cards are valued at ten). So it makes sense to double when you have 9, 10, or 11 (depending on the dealer's up card).
If you've watched Swingers, you probably think that you always double down on 11. The simple rule is this: if you're playing with the rule that the dealer always stands on 17 (including soft hands, such as an Ace and a six), then double down on a 10 or 11 when the dealer's up card is less than your total. If you're playing a dealer who hits on hard 17's, then also double down your 11 vs. a dealer's Ace.
There are other times you want to double down, but this is just meant to give you an idea how it works. Consult How to improve your chances of winning at blackjack for much more detailed information.