Blackjack Strategy When Splitting a Pair of Fives
Split: Never split a pair of fives.
Double Down: When the dealer has a 2 through 9.
Hit: When the dealer has a 10 or Ace.
Why not to split a pair of fives
This pair is a trap. The basic strategy assumes that if you were to split the 5's, you would get two 15's because of all the tens in the deck. 15 is a terrible hand to have because if you hit, you would bust and if you don't, you would have a low score. Why split when you have a total of 10 with a pair of fives? In fact, you should use the strategy for a hard 10.
Why to double down on a pair of fives
You don't split the pair of fives, but instead use their total of 10 and play with that. Because of the assumption that you will get a 10 on the next draw, your total would then be 20, which is extremely powerful. It is very hard for the dealer to beat this kind of score if his upcard is a 2 though 9. So you want to take advantage of a double down here.
Why to hit on a pair of fives
When the dealer has a 10 or ace showing, it is mathematically and obviously the best strategy to just hit on a pair of fives (which total 10). The reason behind this theory is that the dealer also has a very good chance of getting a ten and totaling 20 or 21. You would only be looking at a high probability of a tie or a loss, so it's not worth doing a double down in this case.