Well you can still only double down once, but you can do it an any time. You also get paid more for having 6-7-8 (even more when suited), 5, 6, or 7 card 21, dont have to wait to push the dealer when you get 21, oh, and there are no 10's in the deck. Literally the 10 card, other face cards are still in play as 10.
I was reading about it
here where it says:
In some casinos, players may double double down, or redouble up to two times after doubling down. For example: The player bets one unit and is dealt 2-3, giving a hand total of 5; the dealer is showing a 6. The player doubles the first time and draws a 3. The hand total is now 8 and the total amount wagered is two units. The player doubles a second time and draws a 3. The hand total is now 11 and the total amount wagered is four units. When the player doubles a third time on 11, the total amount wagered will be eight units. Redoubling is a profoundly player-advantageous rule, when optimally executed.
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