As long as Snorlax is Asleep, your opponent's Active Pokémon can't retreat.
If Snorlax is Asleep, this attack does 20 damage plus 30 more damage. (This attack can be used even if Snorlax is Asleep.)
Flip a coin. If heads, this attack does 40 damage plus 40 more damage. If tails, Snorlax is now Asleep.