If the Defending Pokémon tries to attack during your opponent's next turn, your opponent flips a coin. If tails, that attack does nothing.
If the Defending Pokémon doesn't have a Char counter on it, flip a coin. If heads, put a Char counter on it. A Char counter requires your opponent to flip a coin after every turn. If tails, put 2 damage counters on the Pokémon with that Char counter. (Char counters stay on the Pokémon as long as it's in play.)