By adding the included -1 cards to the deck, both the bidding and the trick-taking become more challenging.
The biggest changes are to the setup of Magic Trick. Use the number of suits in the base game rules. Then, shuffle the -1s of the chosen suits in with the playing cards from Magic Trick. Then set one random card of each suit aside without looking at the front. Then proceed as normal.
The cards make gameplay and scoring more challenging. When sorting the cards for your neighbor, remember that -1 is less than 0. Otherwise, gameplay is the same. Each captured -1 is worth minus one point. If a player bid with a -1 card (i.e. if that is the card up their sleeve), they will miss their bid, since it is not possible to take negative tricks. Do not use the Prestige Bonus when playing this way.
—description from the designer