Your opponent switches the Defending Pokémon with 1 of his or her Benched Pokémon, if any; then you switch your Active Pokémon with 1 of your Benched Pokémon, if any. (Do the damage before switching the Pokémon.)
Flip a coin. If heads, the Defending Pokémon is now Paralyzed. If tails, the Defending Pokémon is now Poisoned.