The Power 2s are the JJDD idea that trips up every Ace-High player: two cards that used to be the weakest in the deck are now the third- and fourth-highest cards in the game. The 2 of Diamonds and the 2 of Spades sit right under the Little Joker. Above the Ace of Spades. Above everything else.
The exact ranking
Top to bottom of the trump line: Big Joker, Little Joker, 2♦, 2♠, Ace♠, King♠, Queen♠, Jack♠, 10♠, 9♠, 8♠, 7♠, 6♠, 5♠, 4♠, 3♠. That's 16 trump cards in total. The two Power 2s and two Jokers make up the top four.
Everything else in the deck — the Ace of Diamonds, the Ace of Hearts, the Ace of Clubs — plays by normal high-card rules within its own suit and gets cut by any spade.
Why the 2 of Diamonds is special
The 2♦ counts as a spade for trumping purposes but plays as a diamond when diamonds are led. That means it can win a diamond trick from a low position and also cut any non-spade lead when you're void in the suit.
It's a card that does two jobs. It's why leading diamonds against a good JJDD player is dangerous — the 2♦ could be sitting in any hand at the table.
What the Power 2s mean for bidding
If you have a Power 2, count it as a guaranteed book. Full stop. The only cards that beat it are the two Jokers — combined odds you're up against both are low, and even then you'll usually have another winner to make up for it.
If you have neither Power 2 nor either Joker, your Ace of Spades is not a lock. It could be beaten four different ways. That's the trap Ace-High players fall into constantly. Bid the Ace as a 'likely' book, not a 'guaranteed' one.
