Once during your opponent's turn, if Qwilfish is your Active Pokémon and is damaged by an attack (even if Qwilfish is Knocked Out), you may flip a coin. If heads, the Attacking Pokémon is now Poisoned. Put 2 damage counters instead of 1 on that Pokémon between turns.
Flip a coin. If heads, the Defending Pokémon is now Paralyzed.