Blackjack Strategy When Playing a Soft 17
Double Down: When the dealer upcard is 3, 4, 5 or 6.
Hit: When the dealer has anything else showing.
Why to double down on soft 17
Soft 17 is not a bad hand but it is not a great one either. The basic blackjack strategy assumes that there are more tens in the deck so it's likely that you would just double down and get another ten card, making this a hard 17 hand. The fact that you should double down here becomes apparent when the dealer is showing a 3, 4, 5 or 6. This combination would likely make the dealer bust if a bunch of tens were drawn, so you have a lot of odds working for you here.
Why to hit on soft 17
When the dealer is showing any other card, then the chances of the dealer busting goes down as he needs to stand on a 17. Because you are holding an ace and have a soft hand, then it doesn't hurt to hit on a soft 17 either since you cannot bust. Basic strategy in blackjack says the best move is to hit in order to get a better hand when the dealer is holding anything else.