Jacks take cards away

Knock a selling route out of range.

Your opponent's middle route is at 24 and sells against your 18. A Jack removes the number card it is played on, with everything attached to it.

Play the Jack on one of the cards in your opponent's middle route.

Their route fell below 21, so it no longer sells. Jacks are just as useful on your own routes, to take back a card that pushed you over 26.