Torment prevents the target from selecting the same move for use twice in a row. If a Pokémon has selected the move it used last turn and becomes affected with Torment, it may still use the move this turn (even if it moves after the user of Torment).
If the target can only use one move (e.g. is holding a Choice item, only has one move with PP, is under the effect of Encore, etc.), the target will be forced to use Struggle every second round. Struggle is not affected by Torment; if a Pokémon has run out of PP for all of its moves, it can continue to use Struggle. Moves such as Rollout will still be fully executed under the effects of Torment.
Instead of prohibiting a move from being used twice in a row, Torment seals the last move the target used. If a user's move is sealed by Torment, it will remain sealed until the user moves to the next floor.