On the turn that Dig is selected, the user will dig underground, On the following turn, Dig will do damage, PP will be deducted from it, and it will count as the last move used. Once Dig is selected, the user will not be able to switch out until it is disrupted or fully executed.
If Dig is not fully executed, PP will not be deducted from it, and it will not count as the last move used. If the opponent uses Mirror Move during the turn that the user digs underground, Mirror Move will copy the move that the user executed immediately before using Dig (or fail if it cannot).
Full paralysis and self-inflicted damage due to confusion will disrupt Dig. Additionally, if the user is fully paralyzed during the mostly-invulnerable turn of Dig, the semi-invulnerable part will not be reset until the user switches out or fully executes Dig.
The user now cannot avoid Earthquake and Magnitude during the semi-invulnerable turn and will receive double damage from them, unless the digging Pokémon would not receive damage from the moves normally (i.e. Flying-type Pokémon or those with the AbilityLevitate). Fissure may now hit even during the semi-invulnerable turn of Dig.