Pokémon Gold Version, Silver Version, and Crystal Version - Two-Turn Attacks
In a two-turn attack, the user prepares for the attack on the first attack segment, and hits the opposing Pokémon on the second attack segment. The user can’t take any action until the user finishes using the two-turn attack. PP is deducted on the first attack segment of the two-turn attack. Two-turn attacks work by checking the last move the user chose for use on the turn after the first attack segment of the attack; if that move is a two-turn attack, the second attack segment of the attack is done. If the user is prevented from using the two-turn attack on that turn, its effect ends. If the user tries to use a two-turn attack with Mirror Move or Metronome, it fails if the user is asleep or frozen.
A two-turn attack is not a multi-turn attack, and vice versa. A two-turn attack remains a two-turn attack even if the attack would not take two turns.
For all two-turn attacks, on the first attack segment of the attack, no accuracy check is done and no check is made to determine whether the attack will be ineffective against the opposing Pokémon or otherwise be avoided by the opposing Pokémon.
The phrase "if the opposing Pokémon is using Dig/Fly" means "during the effect of Dig/Fly for the opposing Pokémon." For Mimic, it can copy Dig or Fly after the opposing Pokémon had used it to deal damage. The returned attack from Future Sight will miss the user while it is using Dig or Fly.