Blackjack Strategy When Playing a Hard 9
Double Down: When the dealer upcard is 3, 4, 5 or 6.
Hit: When the dealer is showing anything else.
Why to double down on hard 9
The most important rule in basic strategy is the ten cards are most likely to be dealt in blackjack. You want to see the dealer have a 6 because if he has a ten, then the dealer will have to hit on a 16 and will likely go bust. Mathematics show that the dealer goes bust when he has 3, 4, 5, and 6 showing. The player has an overall better game when he doubles down on these opportunities in the long run. Also, when you are playing with a hard nine (9), you have a high probability of getting a nineteen (19) on a hit, which is a powerful hand. You essentially have two major blackjack strategy advantages working for you at this point at the same time!
Why to hit on hard 9
When the dealer has an ace or anything other than above, then the player odds go down and it is just best to settle for a hit on a hard 9 and hopefully get a 19. This is indeed a nice hand so there is nothing to complain about there, but the dealer just has better chances of matching that or better.