Every time you manage to pick six cards in a row, you receive a bonus for the uninterrupted sequence – an additional card in the deck. The number of cards in the current sequence is displayed in the upper-right corner of the screen.
The sequence is not interrupted by the use of the undo tool, but if the move that has been undone is picking a card, then the number of cards in the sequence is decreased. Using the joker tool does not interrupt a sequence, but the joker does not count toward the sequence. However, if you pick a joker from the field, it does count toward the sequence and, obviously, does not interrupt it. Picking a key card to unlock locked cards does not break the sequence and does not count toward it.