The CHAINED card is introduced at level 51.
To unlock the chained card you need a run of 4 cards.
Each time you remove a card one part of the chain will vanish until the forth card the last part will disappear and you can remove the card or cards.
BUT if you DON'T remove the card before you turn over another from the stack, the chain will return!
Your run can be longer than 4 cards as long as you don't turn over another card from the deck.
This is a good time to use your joker if the chained cards don't match your run.