Poison affects life points.... Explanation:
Round 1: A freak manages to attack your opponent while their bonus is active.
Round 2: Nothing happens yet
Round 3: If the opponent has more than 3 hp, he takes 2 damage
Round 4: Same as Round 3
Then when all the rounds are over, if your opponent still has more than 3 hp, he'll take another 2 damage before the winner is decided.
In other words, if you were able to play Vassili successfully, your opponent would take 5 damage, then another 2 in round 3, then 2 in round 4. That's a total of 9 damage.
If you use a card with less damage than that, they would receive 6 additional damage because poison will take effect again just before the winner is decided.