The house rules for blackjack at most casinos allow players to "double for less." Simply put, a player can double a hand for an amount less than the amount of their initial wager. Doubling takes place when the player has a big advantage on a hand. In exchange for only receiving one more card, the player is allowed to increase his wager. Wagering anything less that the full double is equal to giving back some of the player's expected win amount. In short, it is dumb for a player to double for less, so the house is glad to allow players to do so.