Blackjack Strategy When Playing a Hard 10
Double Down: When the dealer upcard is between 2 and 9.
Hit: When the dealer is showing anything else.
Why to double down on hard 10
Ten cards are very nice to have in blackjack. The basic strategy always says that there are more tens in play than any other card. This means when you have a hard ten, then you are likely to get another ten. This would put you up at 20 points, which is an extremely powerful hand. You want to double down when the dealer has anything between 2 and 9, especially when he has a 7, 8 or 9 because the dealer must stand and the player automatically wins with the 20.
Why to hit on hard 10
When the dealer has an ace or a ten card showing, the dealer can tie or actually beat a 20. The worst card the dealer could have is an ace because basic strategy says there are more tens available and the dealer would be more likely to get a blackjack. If the dealer is showing a 10, it's much better because you could just tie the dealer and the bet would result in a push and your bet is returned, which is better than losing it.