To double for less than your initial wager
To double for less than your original bet. Not a recommended move. To take an exaggerated example, if you had a 1000 dollar bet up, would you double down for 1 dollar? You are taking all the risk of getting only one card, bet as much as you can.
Doubling down with less than two times the original bet. Typically, when doubling is permitted, the player is not required to literally double his bet, but may raise it by any amount up to (but not more than) the original bet.
To double down with less than two times the original bet. When doubling is allowed, the player does not have to actually double his bet, but may increase it by any amount up to the original bet.
Similar to double down, but this time players may only place a bet less than that of their original bet.
When allowed, same as double down, except that the player can place an additional bet for less than the original wager.
To double less than your original bet when in ad advantageous situation.