No more collaboration! You and your opponent each have a draw pile to use up. You can do this by placing your cards on your stacks or on your opponent's stacks. But the game is Machiavellian: on your batteries, you respect your usual rules. On the other hand, a card placed on an opponent's pile must necessarily help your opponent.

So why play on the opposing stacks? Because the game is evil: to rebuild your hand is your only way.