The jack leadback in Double Deck Pinochle
In Double Deck Pinochle, when you're in the lead (you play the first card of the trick), always look for a way to pass the lead to your partner. If you can, they get a chance to take tricks with any Aces they have. That's a nice coup. If you play a low card, chances are good that your left-hand opponent will capture the lead with an Ace. A good player watches their partner's cards for a sign that the lead can pass straight to them and skip the left-hand opponent.
In Double Deck Pinochle, Aces are the strongest cards, so players keep them to use when leading a trick. If you lead an Ace and your partner plays an Ace of the same suit in that trick, it really stands out. When this happens, it's usually a "leadback". Your partner is telling you "If you exit in this suit, I'll take the lead". Either they have the last remaining Aces in the suit or they have no more cards of that suit and can take the lead by trumping in.
The ace-on-ace leadback is an excellent signal, but it isn't always possible. The jack leadback is another option. If your partner plays a Jack of the same suit in a trick where you're leading, they may be trying to tell you to exit in that suit. It isn't as reliable as the ace-on-ace leadback. Sometimes your partner just has no choice but to play a Jack (they have nothing left in that suit except Jacks). And some players don't use the jack leadback, probably because they don't know about it.
So how does the jack leadback work? Usually, when your partner is in the lead, you try to give them low point cards (Kings and sometimes 10s). Once you run out of point cards, it seems obvious to play your lowest cards, which means any Jacks you have. Instead, give Queens to your partner. Avoid playing Jacks, because they would signal your partner to exit in that suit. If you have Jacks, try to play them when an opponent has the lead. If you can, keep one Jack in your hand in the hope that you can use it as a leadback when your partner takes the lead. Only keep that Jack if you actually have an Ace in that suit! Otherwise, get rid of your Jacks when an opponent is leading. There's no point in a leadback if there's no chance you'll have the last high card of that suit.
The bots at World of Card Games read your jack leadback and use it themselves when they can.
Video tutorial of the jack leadback
This video shows the jack leadback in a hand of Double Deck Pinochle.
Resources
There's a fun Double Deck Pinochle tutorial at World of Card Games. You can also find Double Deck Pinochle tips at Pagat.