My friends and I would sometimes play un-cards in commander. We had a dispute on what Checks and Balances should do and there is no oracle or rulings. One friend wanted to play it more like it was worded. "agrees to choose and discard a card" meaning you could agree while empty handed and still counter the card. This made the game virtually unwinnable because you'd always be able to counter anything that won the game. I'd like your official input on how the card works.
As the Un-Rules Manager, let me give you the official ruling. All opponents have to actually discard a card to counter the spell.