The Dark Side has different Action Cards with much stiffer penalties. You start playing with the Light Side, but whenever someone plays a FLIP card, the deck is flipped over (as are the cards in your hand) and now everyone must play off of the Dark Side of the cards.
UNO FLIP plays like regular UNO, except there are two sides to the deck of cards: a 'light' side and a 'dark' side.
