Doubling down gives you the chance to double your initial bet and can only be made with a two card hand. In order to do so simply double the stakes on the table. In online blackjack casinos you have a "Doubling Down" button which you can click that will perform the same action.
Doubling your initial bet causes the dealers to draw a third card After that, the dealer will draw a 3rd final card to your hand. You can not hit again after you double-down.
In blackjack the doubling down option is very useful when you are sure that all you need is one more card. Rarely do casinos allow you to double after splitting, but blackjack variations that do offer this option are mainly found in online blackjack casinos.
Notice that some casinos only allow you to double on hard hands, not on soft ones, such as 6/A. Make sure to check the game's rules before you place money on the table.
There are numerous guides for doubling down strategy which rely on different odds and blackjack strategies, these are worth reading as they would explain to you when to double down with your cards.
Article contributed by Sam Marshall 30th of november '05