In Double Deck Pinochle, when you are in the lead - i.e. playing the first card in the trick - you should always look for a way to pass the lead to your partner. If you can do this, giving them a chance to take tricks with any aces they have, it is a nice coup. If you play a low card, chances are good that your LHO (left-hand opponent) will capture the lead with an ace. A good player will be looking at their partner's cards for a sign that the lead might be passed directly to them, skipping the LHO.
In Double Deck Pinochle, aces are the strongest card, so players keep them to use when leading a trick. If you lead with an ace, and your partner discards an ace of the same suit in that trick, it really stands out. When this happens, it is usually a "leadback": your partner is telling you "if you exit in this suit, I will take the lead". Either they have the last remaining aces in the suit, or they have no more cards of that suit and can gain the lead by trumping in.
The ace-on-ace leadback is an excellent signal, but it is not always possible. The jack leadback is another option. If your partner plays a jack of the same suit in a trick where you are leading, they may be trying to tell you to exit in that suit. It is not as reliable as the ace-on-ace leadback. Sometimes, your partner just has no choice but to play a jack (they have nothing left except jacks). And some players do not use the jack leadback; they are probably unfamiliar with it.
So how is the jack leadback accomplished? Usually, when your partner is in the lead, you try to give them low point cards (kings, and sometimes tens). Once you run out of point cards, the obvious thing seems to be to play your lowest cards - any jacks, if you have them. Instead, you will want to give queens to your partner. Avoid playing jacks, which would signal to your partner that they should exit in that suit. If you have jacks, try to discard them when your opponent has the lead. If possible, keep one jack in your hand in the hopes that you can use it as a leadback to your partner when they take the lead. You will only want to reserve that jack if you actually have an ace in that suit! Otherwise, get rid of them when the opponent is leading. There is no point in playing a leadback if there's no chance that you will have the last high card of that suit.
As of June, 2017, the World of Card Game "bots" will read your jack leadback, and also use it themselves, if possible.
This video demonstrates the jack leadback in a hand of Double Deck Pinochle.
Here are some links to discussions about the jack leadback:
There's a fun Double Deck Pinochle tutorial at World of Card Games. You can also find Double Deck Pinochle tips at Pagat.