Doubling down is one of the hallmarks for a successful blackjack strategy.  By using this option at the right times, a player can walk away with twice as much money on a single bet.  Players simply put down another wager equal to their original bet (some casinos will allow players to put down a wager less than the original) when circumstances allow.  All casinos have different rules as to when a player can double down, although most online casino sites generally follow the same practice.  While each situation is unique (according to the Basic Blackjack Strategy) the most opportune times to double down are when your hand is showing a total of 10 or 11, and the dealer is showing a low value card.  In the case of Spanish 21, the rules of doubling down vary somewhat from straight online blackjack.  The only time you are permitted to double down is after you have taken one or more hits on your hand, or, after you have split your hand.  Keep in mind that if you double down, you will not qualify for any 21-hand bonuses.